Correct answer:
Emily: ¿cuántos tienes?
Rosa: Tengo cuatro.
Emily: OK.
¿cuántos tienes? translates into how many do you have? because they are talking about countable things and we know this because of the reply of Rosa that says I have four (tengo cuatro). So cuántos for a number of things. On the other hand, tengo comes from the verb tener and is its conjugation for the first person singular in the simple present.
