Answer:
B. 9 : 21
Step-by-step explanation:
Multiply numerator and denominator by 3:
... 3/7 × 3/3 = 9/21 ⇒ 9 : 21
_____
Any denominator that is not a multiple of 7 will not work. 35 is a multiple of 7 (=5×7), but 12 (=4×3) is not the same multiple of 3.
