"Tú nos llamas muy a menudo."
Translation:You call us very often.
"You call us quite often" seems more appropriate. Very often is super awkward.
Nos is the direct object pronoun here. One of the rules for object pronouns is that they go immediately before the conjugated verb (not after them).
The "A" is superfluous.
When "nos" is acting as an indirect object, it means "to us" all by itself.
I don't know why pretty often is a wrong answer, I know it's not a literal translation but it's pretty (very) much equivalent