Yes, just stress. They are interchangeable. See my comment to Shannon at https://forum.duolingo.com/comment/27148270/They-are-small-animals and several similar comments, but for the question they are pretty much the same.

You cannot use jak in this exercise. As Renardo_11 has pointed out, jak is an adverb generally meaning "how." Co is a pronoun generally meaning "what."

There is no Czech word “yak”, as “y” is not used in the Czech language, except for foreign names.

“Jak” is a comparative and interrogative adverb roughly equivalent to the English “how.” I now can't think of a situation where it would mean “what.”

Since “jak” is an adverb it is not normally used with být (to be); I think you would have to use the neuter form of the adjective, jaké: “jaké to je?” = what is it like? (Maybe my word order is wrong here, and “jaké je to” should be prefered.) But that does not mean that “jaké” means “what;” it means “what … like” or “like what.”