The correct option to complete the sentence is "that was the girl I told you about." This option is correct because it uses the correct grammar rules, with the object pronoun "you" before the verb "told." The sentence means that the boy had previously informed his mother about the girl, and he is now identifying her to his mother. The other options have grammatical errors, such as the use of an incorrect pronoun, a misplaced preposition, or a wrong verb tense, which makes them incorrect.