Choose the correct structure to complete the sentence. __________ on his own,…

The result clause uses “would have been forced to dismiss,” which is the perfect-conditional form. This signals a third (past unreal) conditional, so the blank must supply an if-clause referring to an unreal past condition.

In formal English the if-clause of a third conditional can be inverted by dropping “if” and moving the auxiliary “had” before the subject: “If he had not resigned” → “Had he not resigned.” The negative “not” stays after the subject.

So the complete sentence reads: “Had he not resigned on his own, they would have been forced to dismiss him from service” = “If he had not resigned on his own, they would have been forced to dismiss him.”

A simple past-perfect main clause (“He had resigned / He had not resigned”) cannot join the result clause without “if”, producing two independent clauses with no conditional link. “Unless he had not resigned” is wrong both in form and meaning: “unless” already means “if not,” so adding “not” creates a double negative.