An introductory adverb clause is usually followed by a
semicolon
colon
period
comma

The answer is that an introductory adverb clause is usually followed by a comma. This is to introduce a pause to separate the two clauses. For example, When you go to the beach, you like to play with the sand and build sand castles. There you see that the indroductory clause, "when you go to the beach" is separated from the main clause "you likde to play with the sand and build sand castles" by a comma.

Not b or c so that leaves you with a or d. a is the better choice.