Select the most appropriate option to fill in the blank and make a grammatically correct sentence.

I would have forgotten my appointment if Susan ________ reminded me about it.

hadn’t

The most appropriate option to fill in the blank is hadn’t.

The sentence is in the past subjunctive mood, which is used to express a hypothetical situation. In this case, the hypothetical situation is that the speaker would have forgotten their appointment if Susan hadn't reminded them about it. The past subjunctive form of the verb "to have" is "had," so the correct form to use in the blank is "hadn't."

The other options are not grammatically correct.

• Wouldn’t is the past tense form of the verb "to want." It is not the correct form to use in this sentence, because the speaker is not talking about something that they wanted to do.
• Haven’t is the present perfect tense form of the verb "to have." It is not the correct form to use in this sentence, because the speaker is not talking about something that they have done.
• Had is the simple past tense form of the verb "to have." It is not the correct form to use in this sentence, because the speaker is not talking about something that they did.