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).
Remember that prepositions are direction words. “Above, after, along...” so D is the best fitting answer. Hope this was helpful, if so, please rate my answer as ‘the brainliest’
