The correct answer is A, DoS. If a Python API allows unlimited file upload size, an attacker can upload extremely large files or repeatedly upload many files until server resources are exhausted. This can consume disk space, memory, CPU, application worker threads, temporary storage, or bandwidth, causing the API or hosting server to slow down, crash, or become unavailable to legitimate users. CEH web application security guidance emphasizes validating input and applying file-upload controls such as content-type checks and malware scanning. It also notes that system resources can be depleted, such as by filling disk space, leading to denial of service. In CEH DoS concepts, a denial-of-service attack restricts or blocks authorized users from accessing system resources; flooding servers or exhausting resources prevents genuine requests from being served. XSS involves script injection, SQLi targets database queries, and CSRF forces authenticated users to perform actions. The issue here is resource exhaustion, so DoS is the best answer.

The correct answer is C, tcpdump. Passive OS fingerprinting identifies the operating system of a target by observing and analyzing network traffic without actively sending probe packets to the target system. Tools such as tcpdump capture packets traversing a network and allow analysts to inspect characteristics like TCP window size, Time-To-Live (TTL) values, TCP options, and packet behavior that can reveal the operating system in use. Because the target is not directly interrogated, passive fingerprinting is stealthier and less likely to trigger alerts.
Nmap is primarily associated with active OS fingerprinting because it sends specially crafted packets and analyzes the responses to determine the target operating system. Ping is used mainly to verify host availability, while tracert (traceroute) identifies network paths and intermediate hops between systems.
Neither ping nor tracert performs OS fingerprinting.
In CEH reconnaissance and enumeration topics, passive information gathering focuses on observing existing communications rather than generating new traffic. Therefore, among the given options, tcpdump is the most appropriate tool for passive OS fingerprinting because it captures and analyzes network traffic without directly probing the target.

The correct answer is C. 51% Attack.
A 51% attack occurs when one entity or coordinated group controls the majority of mining, validation, or computational power in a blockchain network. With majority control, the attacker can influence block creation, reorganize recent blocks, reverse transactions, and perform double spending.
CEH-aligned blockchain material explains that blockchain relies on distributed validation and that more than half of the participating systems must agree before a block is accepted into the chain . The scenario states that a single coordinated entity controlled a dominant share of computational power and used that influence to replace confirmed transactions with alternate versions, which is the core behavior of a 51% attack.
Option A. Eclipse Attack is incorrect because an eclipse attack isolates a victim node from the honest network and controls the information it sees.
Option B. Finney Attack is incorrect because a Finney attack is a specific double-spend technique involving pre-mined blocks, usually by a miner, but the scenario emphasizes majority control of validation power.
Option D. DeFi Sandwich Attack is incorrect because sandwich attacks involve transaction ordering/front- running around a victim's trade, not block reorganization through majority hash power.
Option C. 51% Attack is correct because disproportionate control over block creation enabled transaction reversal and double spending.
Therefore, the best answer is C. 51% Attack.

CEH v13 emphasizes that IoT devices must implement secure firmware update mechanisms that enforce authenticity, integrity, and version control. A critical lapse occurs when devices allow rollback to older firmware versions or accept updates without cryptographic validation. This opens the door to "firmware downgrade attacks," where attackers intentionally install outdated but vulnerable firmware to reintroduce exploitable weaknesses. CEH identifies this as part of insecure update design, one of the most dangerous IoT vulnerabilities because firmware governs device behavior, network communication, and trust boundaries.
Without signature verification, integrity checking, and anti-rollback enforcement, attackers can load malicious or deprecated firmware to escalate privileges or pivot deeper into a network. Options B and C represent different categories of IoT weaknesses, and D refers to supply-chain issues, none of which match the described rollback exploitation. Therefore, the vulnerability demonstrated is the absence of secure update mechanisms.

The most clearly demonstrated vulnerability is Lack of authentication. In CEH-aligned network device security principles, authentication is the fundamental control that verifies a user or device is permitted to access administrative functions or participate in trusted communications. The scenario states that the router allows external administrative access without requiring a password. That directly indicates that authentication is not being enforced for management access, meaning an unauthorized user could potentially gain administrative control simply by reaching the management interface.
The second observation reinforces the same core weakness in a different context: the router uses a protocol that provides neither encryption nor validation. In CEH terms, "validation" in routing and management protocols commonly refers to authentication and integrity checks, ensuring that updates or communications are sent by trusted peers and not altered in transit. When a protocol lacks validation, an attacker may be able to inject rogue updates, impersonate a trusted neighbor, or manipulate routing behavior, depending on the protocol and topology. While this could also be described as "insecure routing protocols," the question asks what vulnerability is most clearly present based on both observations together. The common denominator is the absence of authentication controls: no password for admin access and no validation for device-to-device protocol exchanges.
"Lack of password protection" addresses only the first issue and is narrower. "Firewall vulnerabilities" is not evidenced. Therefore, the clearest and most comprehensive vulnerability indicated by the observations is Lack of authentication.