Answer: Escríbela
“carta” is feminine. That’s why it’s “Escríbela” instead of “escríbelo”
It’s similar to the sentence “Escribe la carta.” Which it’s sort of a “formal” command. It’s sort of like a quick way to give the command. So since the example just says to act upon the verb (comer) then it’s the same verb but conjugated to informal. But it really depends what the sentence is saying. But some verbs that would be also used as inducers objects would be like, “Tómalo”, “Dámelo”, “Míralo/Mírala”, etc.