What should be added when an adverb begins a sentence?
a. a period
b. a semicolon
c. an exclamation mark
d. a comma

Let's take an example.  An adverb is basically an adjective for a verb: it describes a verb, and often ends in -ly.  Here's a sentence, WITHOUT PROPER PUNCTUATION: "Slowly Anna walked."  What fits here?

Let's work backwards.  If D is correct: "Slowly. Anna walked."  This is incorrect, because this would make "slowly" part of a separate sentence, not the beginning of the same sentence.  This is the same situation for C.  If it were correct: "Slowly! Anna walked."  This makes it a separate sentence, so C is incorrect.  What about B?  "Slowly; Anna walked."  This is incorrect because it makes "slowly" too separate from "Anna walked."  Semicolons are used for completely separate ideas, NOT adverbs.  The only right answer is A: "Slowly, Anna walked."  This separates the adverb enough so it doesn't confuse, but you still know we are really saying "Anna walked slowly."

A: a comma