Answer: "Marcos cerrará la puerta" or "Marcos va a cerrar la puerta"
Step-by-step explanation:
I think both can be used within this context, so you could pick either one. I have been speaking Spanish for as long as I can remember, so I know for a fact that both are correct/can be understood.
Nevertheless, I also know that sometimes teachers like you to use a certain form or phrase within the sentence, so I'd suggest picking the one that is best for your situation.
Here are the two options that I came up with:
1) "Marcos cerrará la puerta"
or
2) "Marcos va a cerrar la puerta"
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If anyone would like to comment their opinion, feel free to do so below.
Hope this helps you out a bit!