Answer:
No coman tanta grasa.
Step-by-step explanation:
The correct command is "No coman tanta grasa" because both Renata and Gustavo ate very unhealthy and if you use the command selected you recommend them to eat better.
Commands are used when you tell someone to do something or not to do something, depending if they are affirmative or negative commands. In this case, the sentence is a negative command.