The correct answer is D. was cutting.
The sentence “We saw him cutting a tree yesterday” is in the past progressive tense. The past progressive tense is used to describe an action that was in progress at a specific time in the past. In this case, the action is “cutting a tree” and the specific time is “yesterday.”
The other options are not correct because they do not describe an action that was in progress at a specific time in the past. Option A, “cutting,” is in the simple present tense. The simple present tense is used to describe an action that happens regularly or repeatedly. Option B, “cut,” is in the simple past tense. The simple past tense is used to describe an action that happened at a specific time in the past. Option C, “to cut,” is the infinitive form of the verb “cut.” The infinitive form is used to express a general idea or purpose.
In conclusion, the correct answer is D. was cutting.