Comprehensive and Detailed Explanation:

Cross-site scripting (XSS) is a client-side attack where an attacker injects malicious scripts into a web page viewed by other users. When executed in a browser, it can steal session cookies, perform unauthorized transactions, or execute malicious actions on behalf of the victim.

Option D (Cross-site scripting) is correct because XSS can manipulate client-side input validation to execute unauthorized transactions.

Option A (Privilege escalation) is incorrect because it involves gaining higher privileges on a system, not attacking input validation in a web application.

Option B (DOM injection) is incorrect because DOM-based attacks manipulate browser-side JavaScript but are not necessarily used for unauthorized transactions.

Option C (Session hijacking) is incorrect because session hijacking requires capturing a valid user session, whereas XSS can steal session tokens for this purpose.