Step 1: Understanding the Concept:
This question tests the use of the correct tense for a completed action in the past.
Step 2: Detailed Explanation:
The phrase "a month ago" is a time marker that refers to a specific, finished point in the past.
For actions that were completed at a specific time in the past, the simple past tense is used.
The simple past tense of the irregular verb 'come' is 'came'.
- (A) come - present tense (plural).
- (B) will come - future tense.
- (C) comes - present tense (singular).
- (D) came - simple past tense.
Step 3: Applying to the Sentence:
Since the action happened "a month ago", we must use the simple past tense.
Step 4: Final Answer:
The correct verb is came.