Answer:
Option A.
Step-by-step explanation:
There are two fraction given in this question and we have to write the proportion using colon.
Visually fractions
and
are different but when we simplify these fractions we get the actual relation between them.
We simplify
first.
By dividing from 7 in the numerator and denominator we get

similarly we simplify the fraction 
By dividing numerator and denominator by 3

Now we can say that both the fractions are equal.

So the ratio will be 7 : 21 = 3 : 9
Option A. will be the answer.