Question

Fill in the blank with the most appropriate word.
I would have called him right away if I ____ his telephone number.

• A. have known
• B. will know
• C. had known
• D. would know

Answer 1

The correct option is C had known

Let’s examine the sentence: “I would have called him right away if I ____ his telephone number.” The sentence makes use of the third conditional. The third conditional is used to talk about 'impossible' conditions. They are impossible because they happened in the past and we cannot change them.
It is formed in the following way:
Would have + past participle + if + past perfect
As Option C has verb of past perfect tense, it is the correct answer.