If he ____ a human being, he would not have done this.

Explanation:

Correct Answer is: had been
• Zero Conditional = If + Present + Present (shows scientific and general truth)
• 1st Conditional = If + Present + Future
• 2nd = If + Past Simple + Future in Past ( Sub + would/might/could + Base Form of the Verb)
• 3rd Conditional = If + Past Perfect (had +V₃) + Perfect Modal (Sub + would have/could have/might have+ V₃)

– প্রশ্নে উল্লেখিত বাক্যটির শেষের অংশে he would not have done this থাকায় বুঝা যাচ্ছে এটি Conditional Sentence এর গঠন অনুযায়ী 3rd Conditional হবে৷
তাই শূন্যস্থানে had been বসবে।