The base verb in this sentence is 'see'.
An infinitive verb is a verb with the word “to” in front of it.Thus option B is correct, where we will replace the word 'on' with 'to' in order to make it in the correct form of an infinitive verb. Here 'to' isn't acting as a preposition but is part of the verb 'see'.
The incorrect options are:
See - Saw, would be incorrect in accordance with the tense of the sentence.
Dear - Dearer, would be incorrect as the superlative degree of 'dear' isn't needed as there is no comparison done in the sentence.