The correct option is C would have
Let’s examine the sentence: ‘Wilma _____ called if she had forgotten her keys.’ The sentence talks about an unreal situation that actually did not happen. By reading the sentence, we can deduce that Wilma did not forget her keys. The sentence states the unreal and imaginary situation of what Wilma would have done if she had forgotten her keys. So the sentence makes use of the third conditional.
The third conditional is used to talk about 'impossible' conditions.
Third conditional structure is: Would have + If + past perfect
Now, when we examine the options against the structure, only Option D meets the condition of the sentence with third conditional having the verb ‘would have.’ Hence, Option D is the correct answer.
Options A, B and C cannot be used as they would make the sentence grammatically incorrect.