正解:C
Comprehensive and Detailed in-Depth Explanation:
A passive attack involves monitoring or listening without altering data or system operations. Eavesdropping is a classic example, where attackers intercept data without affecting the transmission.
Keystroke logging (Option A) and phishing (Option D) are active attacks. Piggybacking (Option B) involves unauthorized access and is also active.
ISACA CISA Reference: Domain 5 - Protection of Information Assets