Right answer:
Yo pago en la caja
Since this sentence stands for the first person singular, that is, the subject pronoun of this sentence is yo and given that we need to write a complete sentence to tell what everyone does, then we need to use the simple present tense. This tense is used to talk about habitual actions or routines. This tense is also used to describe things that happen now or in the near future. Therefore, the conjugation of the verb pagar for the first person singular in the simple present is pago. Therefore:
Yo pago en la caja (I pay in the cash register)