Which of the following is a passive information gathering technique?

Correct answer: Reviewing public DNS records

Option A is correct because reviewing public DNS records is a passive technique that gathers information without directly interacting with the target's systems, using only publicly available data sources. Option B is incorrect because a vulnerability scan actively probes target hosts, generating network traffic and log entries on the target. Option C is incorrect because attempting SQL injection is an active exploitation technique that directly interacts with and attacks the target application. Option D is incorrect because running a port scan sends packets directly to the target, which constitutes active reconnaissance and is detectable by the target.