A
Explanation:
If a router learns multiple routes to the same subnet, it chooses the best route based on the metric. This is assuming that the router learned the route from the same routing protocol. If learned from a different routing protocol, then the route with the lowest AD will be installed into the routing table.

Incorrect Answers:
B, C. Only one route to a given destination is placed into the routing table, and the route with the lowest metric is always chosen.

D. This would prove to be a very inefficient method. If this were true, then the entire
routing table would be rebuilt for each router nearly every time an update was received.   _{Your Comment}