Answer: A. When the two populations can no longer interbreed
Step-by-step explanation:
Speciation is a process in which the members of the same population of a species get separated due to geographical, physical, behavioral, and temporal barriers. The separated members develop new traits and becomes a new species.
Among the given options, A. When the two populations can no longer interbreed is the correct option because if the members of the same population will longer interbreed they may experience reproductive isolation from the separated members and with the coarse of evolution they will develop traits which will be responsible for the development of new species. Hence, speciation will complete.