Question

form a compound sentence.
that creature is not a crocodile but is probably an alligator.
even a little bit like
a crocodile, so it must be
a crocodile but is probably
a crocodile, a dinosaur, or

Explanation:

To form a compound sentence, we need to maintain the logical relationship of the original sentence (negating it being a crocodile and suggesting it's an alligator). The option "a crocodile but is probably" fits to complete the sentence as "That creature is not a crocodile but is probably an alligator" which is a compound sentence with a coordinating conjunction "but" showing contrast. The other options either change the meaning (like "so it must be" which is a causal relationship not matching the original contrast) or don't fit grammatically/logically.

Answer:

a crocodile but is probably