Final answer:
The correct completion for the sentence in question, using proper verb tenses, is option (a) are, eat. These verb forms are used to express general truths and habitual actions in the simple present tense.
Step-by-step explanation:
In the context of verb tenses, specifically looking at the simple present and present progressive, the sentence 'Lions, tigers, and cheetahs are in the cat family. Like all cats, they are carnivores, which means they only eat meat, not plants or grass' is correctly completed with option (a) are, eat. This is because when we discuss habits or general truths, such as what certain animals eat, we use the simple present tense. Thus, the correct form of the verb 'be' in this sentence is 'are' as it agrees with the plural subject (lions, tigers, and cheetahs), and the correct form of 'eat' is the base form 'eat,' as it expresses a habitual action or fact.