The correct option is B that
A relative pronoun functions as a pronoun as well as a conjunction. As a pronoun, it refers or relates to a noun that's repeated in both sentences and as a conjunction it combines those two sentences as one. In the given statement, the subject is "I", the verb is "attended" and the object is "the concert". The subject performs the action towards the object. Here, the object is the antecedent of the relative pronoun, because the sentence focuses on the object (an antecedent is a noun which the relative pronoun refers to immediately after in the sentence). From the options, 'that' refers to the 'concert', because the person "I" is talking about the concert he attended. 'That' mostly refers to things, or here, an event. When an object is the antecedent, the relative pronoun is usually implied, 'that' is generally omitted. Hence, option B is correct. "I" is a pronoun that refers to the speaker who attended the concert, it does not relate to the object antecedent, so option A is incorrect. 'Concert' is the antecedent, and not the relative pronoun, it does not imply anything but it's a noun that refers to an event, hence option C is incorrect. 'Whom' is a relative pronoun that refers to people, sometimes it refers to animals, but not things, and here a relative pronoun is to refer to a thing/event "concert". Thus, option D is incorrect.