Step 1: Identify the time reference in the sentence. The phrase “during two years” indicates a period that lies entirely in the past, establishing a past time frame for the action.
Step 2: When an action or experience is completed before another point in the past, the appropriate tense to use is the past perfect tense. This tense clearly shows that the action was already finished before a certain past moment.
Step 3: Recall the structure of the past perfect tense, which is formed as:
had + past participle.
In this case, the correct form is “had had.”
Step 4: The remaining options do not follow this grammatical structure and are therefore grammatically incorrect or impossible.