Correct Answer: DThe first step a penetration tester should take when assessing containers for DDoS attack susceptibility is to scan the containers for open ports. Open ports can serve as entry points for DDoS attacks, thus identifying and securing these ports can prevent attackers from exploiting them to perform a DDoS attack. While other options, like checking encryption settings or the availability of security tokens, are important for overall security, they do not directly relate to DDoS-specific vulnerabilities in the context of containers.