Final answer:
The correct answer is a. nos reimos.
Step-by-step explanation:
The correct answer is a. nos reimos. In this sentence, the subject 'Yo' (I) is performing the action of laughing with the pronoun 'nos' (we). Therefore, the correct verb form is the first person plural of the verb 'reír' (to laugh), which is 'nos reimos'.
Example: Yo y mis amigos nos reimos mucho en la fiesta.
Learn more about Spanish verb conjugation