Animal which eat only plants are:
(A) Parasites
(B) Herbivores
(C) Carnivores
(D) Omnivores

Hint: Nutrition is defined as the uptake of healthy food and balanced diet. Organisms are classified on the basis of mode of nutrition. Some organisms eat plant parts, some eat animals and some eat both plants and animals. There are three types of nutritions: Herbivores, carnivores and omnivores.

Complete answer:
Parasites are those organisms which reside on or in the host. They get their food from the host. This is the case of negative association with hosts. They harm the host. There are two types of parasites:
Ectoparasite: These are found on the surface of the host.
Endoparasites: These are found inside the host.
Parasites are found on both plants as well as on animals. That’s why “Parasite” is an incorrect option.
Carnivores are those organisms which feed on other organisms for obtaining their food. They don’t eat plant parts. Example: Lion.
That’s why “Carnivore” is an incorrect option.
Omnivores are those organisms which eat both plants and animals for their mode of nutrition. Examples: Dog, foxes etc.
That’s why “omnivore” is an incorrect option.
Herbivores are those organisms which eat plant parts only. Example: Deer, birds etc.
That’s why “herbivore” is a correct option.

Note:
Don’t be confused between parasites and herbivores. Parasites are found on plants as well as on animals. Parasites are small sized organisms which are found to be dependent on the host. Herbivores eat plants for their nutrition.