Final answer:
Animals with large masseters eat tough plant tissues, as these robust jaw muscles are suited for the intense chewing needed to grind down fibrous plant parts found in the diet of many herbivores.
Step-by-step explanation:
The large masseters suggest that an animal has powerful jaw muscles necessary for chewing hard or tough food. In the context of the options provided, large masseters are indicative of an animal that eats tough plant tissues, such as those found in stems, leaves, and other fibrous parts of plants.
This is consistent with the dietary habits of herbivores that have evolved to specialize in grinding down plant material, which requires significant chewing action to break down the cellulose-rich plant parts.
When considering the correlation between masseter size and diet, carnivores tend to have sharper teeth for tearing apart flesh rather than large masseters for grinding.
Animals that feed on nectar, berries, or other soft fruit typically have adaptations for sucking or picking rather than for intense chewing. Likewise, animals that consume small organic material carried by moving water or marine feeding generally do not rely on strong masseter muscles to process food.
Therefore, the correct answer to the question "Large masseters indicate that an animal eats:" would be option 'a. tough plant tissues'.