Nessus is the best answer because it is a dedicated vulnerability assessment scanner. Its purpose is to scan systems, services, ports, configurations, missing patches, weak protocols, and known CVEs, then produce a report of discovered weaknesses. That directly matches the requirement to "identify potential security weaknesses" on a target network. Metasploit is mainly an exploitation framework used to validate or exploit vulnerabilities, not the best first-choice vulnerability assessment tool. Burp Suite is primarily used for web application testing, not broad network vulnerability assessment. Wireshark captures and analyzes packets, which is useful for troubleshooting and traffic inspection but does not perform vulnerability assessment. For Security+, Nessus maps clearly to vulnerability scanning and assessment.

Data masking replaces sensitive data with realistic but fictional data, preserving format and usability while protecting confidential information during testing in environments like UAT (User Acceptance Testing).
Tokenization (B) replaces data with tokens but is more common for payment data in production. Obfuscation (C) scrambles data but is less structured than masking. Encryption (D) protects data confidentiality but may not be practical for testing.
Data masking is a best practice for protecting sensitive data in non-production environments covered in the General Security Concepts domain#6:Chapter 7 CompTIA Security+ Study Guide#.

The best answer is C. Data retention.
To avoid unnecessary liability after the end of a legal contract obligation with a third party, an organization should follow a proper data retention policy. Data retention policies define how long data must be kept and when it should be deleted or securely destroyed once it is no longer needed for legal, contractual, regulatory, or business purposes.
Keeping third-party data longer than necessary can increase:
legal exposure
privacy risk
breach impact
storage and governance burden
Why the other options are incorrect:
A). Data encryptionEncryption protects data confidentiality, but it does not address whether the data should still be retained at all.
B). Data classificationClassification helps determine sensitivity and handling requirements, but it does not define when data should be disposed of.
D). Data inventoryAn inventory helps identify what data exists, but it does not by itself reduce liability after contractual obligations end.
From the SY0-701 perspective, reducing liability after contractual or legal obligations end requires proper retention schedules and secure disposal, so C is the correct answer.

Side loading is the process of installing applications from unofficial sources, often bypassing standard app stores. This increases the risk of installing malicious software, such as a rootkit, which is a type of malware designed to provide persistent privileged access while hiding its presence.
Reference:
CompTIA Security+ SY0-701 Official Study Guide, Domain 2.1: "Side loading applications from unofficial sources can introduce malware, such as rootkits, to the system." Exam Objectives 2.1: "Compare and contrast different types of threats."