The best answer is B. Address pool exhaustion . The key detail is that this started happening while deploying a new fleet of computers on a DHCP network . That strongly suggests the number of available leases in the DHCP scope has been used up. When no more addresses are available, new devices cannot obtain valid network settings and will fail to connect properly.
This is a common issue when many systems are added at once. Existing devices may already be consuming most or all of the available leases, and the new clients are left without an address assignment. Without a valid IP configuration from DHCP, the computers will not be able to reach the local network or the internet in the normal way.
The other choices are possible network problems in general, but they are less likely in this exact scenario. An incorrect default gateway or incorrect subnet mask would more often reflect a bad DHCP option or a misconfiguration affecting many clients in a different way. A duplicate IP address can affect connectivity, but it does not naturally match the clue about many new devices being added at once on a DHCP scope.
When a question combines DHCP, many new clients, and sudden inability for new systems to connect, address pool exhaustion is the most likely cause.