The past subjunctive is the hypothetical subjunctive. It is restricted to:

The correct answer is Option D (Were).

Key Points:
• The Subjunctive Mood in English:
• In modern English grammar (as outlined by Quirk, Greenbaum, Leech, and Svartvik), the past subjunctive—also called the hypothetical or unreal subjunctive—survives almost exclusively in the single form 'were'.
• It is used with singular subjects (I, he, she, it) in unreal, imaginary, or counterfactual conditional clauses and wishes where standard indicative grammar would use 'was':
• 'If I were you, I would accept the offer.'
• 'I wish he were here.'
• It does not apply to modal auxiliaries like shall, will, or can.
• Hence, the hypothetical past subjunctive is formally restricted to 'were'.
• Key Takeaway: The hypothetical past subjunctive in modern English survives solely in the form 'were' for unreal conditions.