which the user ‘bob’ belongs?

Which of the following is the quickest way to create a file that lists groups to which the user ‘bob’ belongs?

A.
cd /etc; cat groups | grep bob | vi > bob_groups.txt

B.
find /etc/passwd bob >> bob_groups.txt

C.
grep bob /etc/group > bob_groups.txt

D.
whereis bob >> bob_groups.txt