"They usually work with my parents."
Translation:Oni zwykle pracują z moimi rodzicami.
No. "Najczęściej" (which is a single word) means "most frequently" or "most often".
E.g. "Oni pracują z różnymi ludźmi, ale najczęściej z moimi rodzicami" - "They work with different people, but most often with my parents."
To me, that seems the same, but looking it up, I can see there are fine differences.
I saw it among the three proposed variants and I just wondered if it would work.
"Zazwyczaj" itself would, but watch out, the sentence may have some other mistake - if the word itself was not taught, it's probably not used in a correct option even if it's a synonym. At least that's what comes from my observations of Russian course.
there has been no indication that zazwyczaj is not an acceptable alternative.