Two species of frogs mate with each other. the embryo dies after a few cell divisions. what kind of reproductive isolation does this example represent?

The correct answer would be post-zygotic isolation.

Post-zygotic isolation refers to the set of mechanisms which prevent the formation or production of the viable infant after fertilization.

It includes hybrid zygote abnormality, decreased hybrid viability, and hybrid infertility.

The zygote formed either develops abnormally or it is not viable to develop into an embryo.  

Lastly, if an infant is produced, it would be sterile.

c. establishing proper sleep hygiene