The correct answer is A. The most effective and reliable method for a security engineer to "find reliable IoCs and malware behaviors" is to use Google Threat Intelligence (GTI). When a known indicator like a file hash is identified, the primary workflow is threat enrichment. Google Threat Intelligence, which is a core component of the Google SecOps platform and incorporates intelligence from Mandiant and VirusTotal, is the dedicated tool for this. Searching the hash in GTI provides a comprehensive report on the malware variant, including all associated reliable IoCs (e.g., C2 domains, IP addresses, related file hashes) and malware behaviors (TTPs, attribution, and context). This directly fulfills the user's need.

In contrast, Option D (UDM search) is the subsequent step. A UDM search is used to hunt for indicators within your own organization's logs. An engineer would first use GTI to gather the full list of IoCs and behaviors, and then use UDM search to hunt for all of those indicators across their environment. Option B (Web Search) is unreliable for professional operations, and Option C (manual analysis) is too slow for a "common malware variant" and the need to act "quickly."

(Reference: Google Cloud documentation, "Google Threat Intelligence overview"; "Investigating threats using Google Threat Intelligence"; "View IOCs using Applied Threat Intelligence")