Question

coordinating conjunctions
b o y f a n s
but or yet for and nor so
the whistle blows and the train comes to a stop.
the whistle blows, and the train comes to a stop.
comma no comma

Explanation:

When using a coordinating conjunction (like "and") to connect two independent clauses (each can stand as a sentence: "The whistle blows" and "The train comes to a stop"), a comma is used before the conjunction. In the second sentence, the comma before "and" is correct for this structure.

Answer:

comma