Selected Answer: D

The attacker is exploiting the supply chain vector. The supply chain refers to the series of processes and organizations involved in the production, distribution, and delivery of goods and services. By infiltrating third-party software vendors, the attacker is attempting to gain access to the supply chain and compromise the software that is used by the organization. This can allow the attacker to launch attacks, steal sensitive data, or disrupt operations by inserting malicious code into the software. Social media, cloud, and social engineering are not directly related to the supply chain and would not be effective for infiltrating third-party software vendors.