I am convinced . . . . . . . . his innocence.

The correct answer is: of.

The preposition “of” is used to express certainty or conviction about something. In this case, the speaker is saying that they are certain of the person’s innocence.

The other options are not as appropriate in this context. The preposition “by” is used to express a cause or reason. The preposition “with” is used to express a means or instrument. The preposition “at” is used to express a location or position.

Here are some examples of how the preposition “of” can be used to express certainty

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