In this set
the common thing is that when we break it into the group of two letter, we will observe that no two same alphabet come side by side (as, aa or bb or cc etc....)
secondly in first two set, 'b' is common as "ab-cb"
in next two set 'c' is common as "bc-ac"
similarly in next two set 'a' is common as "ca-ba"
in last two set again 'b' is common as "ab-cb"
hence the answer c is correct