To answer this question, we need to understand the correct verb form to use in the given sentence.
Option A) signing - This option is incorrect because "signing" is the present participle form of the verb, which is not appropriate in this context.
Option B) signs - This option is incorrect because "signs" is the present tense form of the verb, which does not match with the past tense context of the sentence.
Option C) sign - This option is incorrect because "sign" is the base form of the verb, which also does not match with the past tense context of the sentence.
Option D) signed - This option is correct because "signed" is the past tense form of the verb, which is appropriate to describe an action that has already happened.
The correct answer is D) signed. This option is correct because it matches with the past tense context of the sentence, indicating that the person already completed the action of putting their name on the paper.