Answer:
D.
Explanation:
10 and 21 both do not have a common factor which means that a isn't the answer. 6 and 7 also don't have a common factor which means that b isn't the answer. 3 and 5 don't have a common factor which means c isn't the answer. 4 and 6 have a common factor of 2 which means that is the answer. The fraction could be simplified to 2/3.
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