I had forgotten what is the principal part of the verb in bold? The bolded word is forgotten

Assuming your options are present participle, present, past, and past participle, the correct answer is past participle.

Past participle (which can be found in the third column of irregular verbs) is the form of the verb which is used for tenses such as present perfect, or past perfect, in the example above.

Present participle would be - forgetting; present would be - forget; past would be - forgot. Since neither of these is found above, the correct answer is past participle.