Answer:
C. saw
Step-by-step explanation:
The past tense of the verb see is saw while the past participle is seen. See is the present term of the verb.
In the sentence given, there is a referral to something (Paul's action of seeing) that happened in the past and as such the past tense is applicable.
The right answer to fill the gap is saw. As such, the sentence will thus read;
Paul saw his ox, Babe, grazing on the Great Plains. Option C