Mi hermano y yo a la playa en el verano. a. fui c. fuimos b. fuiste d. fue

The right answer is c. fuimos

Whose translation is:

Mi hermano y yo fuimos a la playa en el verano (My brother and I went to the beach in the summer)

fuimos is the conjugation of the irregular verb ir in the preterite tense. The preterite tense (el pretérito) is used to describe actions completed at a point in the past. The most common verbs with irregular preterite forms are ser, ir, dar, and ver. Also, ser and ir have the exact same forms in the preterite.