Concept:The correct form needed is the past participle of the verb "strike".
Explanation:The verb "strike" has three main forms: "strike", "struck", and "struck".
"Strike" is the base form, "strikes" is the third-person singular present form, and "striking" is the present participle.
"Struck" is the past tense and also the past participle form.
When the gap follows an auxiliary verb like "has", "have", or "had", the past participle "struck" is required.
Therefore, among the options given, the correct completion is "Struck".
Answer:D. Struck.