D. Weaponization.
Explanation:
Weaponization is the stage of the Cyber Kill Chain where an attacker creates or modifies a weapon (malware) to deliver it to the target system. In this scenario, the threat actor compiles and tests a malicious downloader to ensure it will not be detected by the victim organization's endpoint security protections. This involves the preparation and testing of the malware to ensure it is effective at achieving the attacker's objectives without being detected by security defenses.
While Reconnaissance (B) involves gathering information about the target organization, and Exploitation (C) involves taking advantage of vulnerabilities to gain unauthorized access, the described actions focus on the creation and testing of the malicious downloader, which aligns more closely with the Weaponization stage.