The correct option is D either ; or
Correlative conjunctions are pairs of conjunctions that work together. Some examples are either/or, neither/nor, and not only/but also. The given sentence requires the use of a correlative conjunction. The combination either/or indicates that there is a choice between the two choices, and only one can be selected. Either is always used with or and it is used to express alternatives and or a choice between two (and sometimes more) things. Thus Option C is correct. Besides, it is mentioned in the sentence that only one ticket is available.The combination neither/nor indicates that neither of the two choices can be selected. In other words, neither choice is available. Thus Option A is incorrect as neither/nor - When they're paired up they negate both parts of a statement while it is inferred in the sentence that only one ticket is available and one of them could go to the concert.The combination both/and indicates that the two items are equally presented and included. Hence Option B is incorrect as it is inferred in the sentence that one ticket is available. Option D can be ruled out as 'either'' is not followed by 'nor' but by 'or'.While 'either' is used to offer a choice between two things,'nor' is used before the second or further of two or more alternatives (the first being introduced by a negative such as ‘neither’ or ‘not’) to indicate that they are each untrue or each do not happen.