My answer is B because it's not a solution, it's just a scenario that can be causing the problem.
I'm not sure if you can tell from the logs what the NAT ACL is (please correct me if I'm wrong), but you CAN tell that the pool only allows for 2 NAT IP addresses: 198.51.100.5 and 198.51.100.6. Additionally, from the logs you can tell that the NAT Pool is exhausted and it's dropping ICMP polls from hosts that don't have a NAT mapping.
Therefore, if you need more than 2 hosts to have access to the outside, you'd need more than the 2-IP NAT pool. "The NAT pool is too small" would be a fine answer if it was a choice, but the next best thing is to say the translation doesn't allow overloading, which would allow more hosts to be translated after the first 2 IPs in the pool are used up.