Answer:
A. What are you talking about, Marcus? Asked Shelby.
Step-by-step explanation:
The correct way of writing the above sentence will be
"What are you talking about, Marcus?" asked Shelby.
As the sentence is based on a conversation, inverted commas should be used to open the sentence as well as close it. The rest of the options are indirect speeches while the first option is a direct speech. Thus, the punctuation.