The correct answer is: C. had been grazing
The sentence is in the past tense, so we need a verb that is also in the past tense. The verb “graze” means to eat grass, and the past tense of “graze” is “grazed.” However, the sentence also tells us that the cow was grazing for some time, so we need a verb that indicates that the cow was grazing for an extended period of time. The verb “had been grazing” is the past perfect progressive tense, which is used to indicate that an action was happening before another action in the past. In this case, the cow was grazing before it did not return home in the evening.
The other options are incorrect because they are not in the past tense or they do not indicate that the cow was grazing for an extended period of time. The verb “grazes” is in the present tense, and the verb “was grazing” is in the simple past tense. The verb “grazed” does not indicate that the cow was grazing for an extended period of time.