The answer is:
A. It rained heavily last night; worms were easy to spot this morning.
Compound sentences consist of at least two independent clauses with related ideas. They can be separated by either a semicolon or a comma plus a coordinating conjunction like and or but.
The rest of the sentences are incorrect because the clauses in B are set apart by a semicolon plus a coordinating conjunction, in C they are divided by a comma, and in D they are separated by a colon followed by coordinating conjunction and.