ক. If he studied more
খ. Had he studied more ✔
গ. Studying more
ঘ. If he were studying more
ব্যাখ্যা: Main clause---এ 'would have been able' রয়েছে, যা Third Conditional নির্দেশ করে। If বাদ দিয়ে Had দিয়ে বাক্য শুরু করে Third Conditional গঠন করা যায় (Had + subject + V3)।