Step 1: Understanding the Concept:
This question tests the correct verb form to use in the present perfect tense.
Step 2: Detailed Explanation:
The present perfect tense is formed with the auxiliary verb 'have' or 'has' followed by the past participle (the third form) of the main verb.
The sentence structure is "We have not ........". 'Have' is the auxiliary verb. We need the past participle of the verb 'to see'.
The forms of the verb 'see' are:
- Base form: see
- Simple past: saw
- Past participle: seen
Therefore, the correct form to use after 'have' is 'seen'.
Step 3: Applying to the Sentence:
The correct completion of the sentence is "We have not seen him for a year."
Step 4: Final Answer:
The correct verb form is seen.