CBC mode introduces dependency between blocks to prevent the pattern leakage seen in ECB. It starts with a random (or unpredictable) IV for the first block. Before encrypting block 1, CBC XORs plaintext block 1 with the IV, then encrypts the result. For block 2 and onward, CBC XORs each plaintext block with the previous ciphertext block before encryption. This chaining means that changing one plaintext block affects that block's ciphertext and also influences the next block's computation. The IV ensures that encrypting the same message twice under the same key produces different ciphertexts (assuming a fresh IV). Option A (ECB) has no IV or chaining. OFB and CFB are feedback modes that effectively generate a keystream; they do use an IV, but the "uses the result to encrypt the next block" wording most directly matches CBC's ciphertext-chaining description in standard teaching. CBC still requires integrity protection (e.g., HMAC or an AEAD mode) because it can be malleable without authentication. Therefore, the correct mode is Cipher Block Chaining (CBC).

Lightweight cryptography is designed for constrained environments-devices with limited CPU, memory, storage, bandwidth, and power (battery). Examples include IoT sensors, smart locks, RFID tags, embedded controllers, and industrial devices. Administrators choose lightweight algorithms and protocols to maintain reasonable security while fitting strict resource budgets and real-time constraints.
The goal is not "weaker security because data is unimportant," but rather efficient security that can still meet threat models under constraints. Option B captures this: embedded systems often cannot afford the computational cost of heavy cryptographic primitives (large key sizes, complex modes, frequent handshakes) or may struggle with latency and energy consumption. Option A is irrelevant because physical security of a desktop doesn't remove the need for cryptography in communications or storage. Option C is the opposite of lightweight design. Option D is a poor justification; security design should be based on risk, and lightweight cryptography is not merely for "minimal protection," but for practical deployability under constraints. Therefore, the correct reason is limited resources on embedded systems.

A true random number generator (TRNG) draws randomness from physical phenomena that are inherently unpredictable and not algorithmically reproducible. Because of this, it is nondeterministic:
you cannot feed it the same "input" and expect the same output stream. TRNGs are often slower than PRNGs because they depend on collecting entropy from hardware sources and may require conditioning to remove bias. This aligns with option B: slow and nondeterministic, producing different results even under similar or repeated conditions. Option A describes a deterministic PRNG, where identical seeds yield identical sequences. Option C is unrelated; factorization is a hard math problem used in cryptography (e.g., RSA security assumptions), not a randomness generator definition. Option D describes a counter, which is deterministic and not random. In secure systems, TRNG output may seed a cryptographically secure PRNG to provide both unpredictability and high throughput; but the defining characteristic of a TRNG is nondeterminism from physical entropy. Therefore, option B is correct.

SHA-1 (Secure Hash Algorithm 1) produces a fixed-size output of 160 bits (20 bytes). Hash output size matters in cryptography because it influences collision resistance and the effort required for various attacks. For an ideal n-bit hash, finding a collision by generic means is expected around 2^(n/2) operations (birthday bound). With SHA-1's 160-bit output, that generic bound would be about 2^80, which was once considered strong; however, SHA-1 has been broken in practice with significantly less work due to cryptanalytic advances, and it is now deprecated for most security uses. Still, the question is strictly about output length, not current suitability. The other options do not match SHA-1: 40 bits would be far too small for a modern hash, 80 bits is not SHA-1's output, and 320 bits would imply a much larger digest (closer to SHA-256's 256 bits or SHA-384's 384 bits). Therefore, the correct SHA-1 output length is 160 bits.

A brute-force attack exhaustively tries every possible key or password candidate until the correct one is found. Because it explores the full search space (or a very large portion of it), brute force is often the slowest method, especially when strong keys, long passwords, rate limits, and slow password hashing (bcrypt/Argon2) are used. By contrast, a dictionary attack reduces work by trying only common or likely passwords, often succeeding quickly against weak human-chosen secrets. Rainbow table attacks shift work into precomputation; once a table exists, lookup can be faster than brute-force-though salt and modern hashing defeat them. Birthday attacks are about finding collisions, not necessarily recovering a specific secret, and their expected work is about 2^(n/2) for an n-bit hash, which can be less than brute-force key search in many contexts. Therefore, among the listed options, brute-force generally takes the longest to succeed because it makes the fewest assumptions and does the most total work.