Final answer:
The sentence is written in the active voice.
Step-by-step explanation:
The sentence 'Monty Burns is enjoying a peanut butter and marshmallow fluff sandwich' is written in the active voice. In the active voice, the subject of the sentence performs the action. In this case, Monty Burns is the subject and he is actively enjoying the sandwich. The passive voice, on the other hand, would emphasize the receiver or target of the action. For example, if the sentence was 'The peanut butter and marshmallow fluff sandwich is being enjoyed by Monty Burns,' it would be in the passive voice.
Learn more about Active Voice