This is punctuated correctly.
"Marshmallows are my favorite snack" is a complete sentence unto itself.
The comma instead of a period shows that the sentence spoken is part of the whole sentence, which includes the speaker (she) and a word to describe how the speaker said it (explained).
Answer:
B. predicate
Explanation:
the underlined portion has a verb and states something about the subject so therefore it is a predicate
The answer is C. a stone monument
