Comprehensive and Detailed Step-by-Step Explanation:
Metrics should provide meaningful insights into the organization's risk exposure and security performance.
Evaluating this option:
A). The number of blocked external attacks is not representative of the true threat profile: This is the BEST answer because counting attacks blocked does not reveal the effectiveness of security controls or the real risk environment.
B). The number of blocked external attacks will vary by month, causing inconsistent graphs: While variability is a concern, it does not make the metric invalid.
C). The number of blocked external attacks is an indicator of the organization's popularity: This is true but irrelevant to assessing the effectiveness of security measures.
D). The number of blocked external attacks over time does not explain the attackers' motivations:
Understanding motivations is useful but not directly tied to evaluating the firewall metric's effectiveness.
Reference: CISM Job Practice Area 2 (Risk Management) emphasizes the need for meaningful and risk-based metrics.