What is one cause of speciation a. populations are morphologically separated b. populations become infertile c. populations interbreed d. populations become reproductively isolated

Your answer should be B/the 2nd option.

:-)

One cause of speciation is A. population are morphologically separated.

This is because, D and B suggest that the population cannot reproduce at all, causing extinction. While C doesn't directly lead to extinction, interbreeding can cause genetic flaws that can lead to infertility and a sped up extinction process. 

Hope I could help.

The higher you go up the less oxygen there is 

Ican't see the images : (