Which stage is the last stage of speciation?
a. the populations become adapted to different environments and eventually become so different that they cannot interbreed to produce fertile offspring.
b. the populations are isolated and thus do not interbreed.
c. the populations become totally separated from one another.
d. the populations interbreed successfully.

The answer is B.
A is not even true generally, species won't stray from their own(especially just for environment) unless under drastic circumstances.
C is somewhat true but not as specific as B, speciation has to do with interbreeding and barriers related to it.
D is obviously just the opposite of what happens with speciation. It is due to either biological or geographical barriers inhibiting breeding that causes speciation to occur.