The correct option is C 're going
A temporary action that may not be happening at the time of speaking takes the present continuous form of the verb.
"Going" in option A is incorrect as with a noun/pronoun an auxiliary verb is also required. Just "going" indicates travelling.
"Are going/'re going" in option B is correct as the present continuous form of the verb needs to take an auxiliary verb. It will take the form of S + is/am/are + (verb + ing) + O.
So, 'we're going' is in tense agreement with 'this summer' and demonstrates present continuous tense.
"To go" in option C indicates a phrase which means "for taking away (food).
"Go" is the base form of the verb 'go' and is incorrect as the tense is incorrect and it also needs an auxiliary verb.
Hence, option B is the correct answer.