correct answer is: b) 21

Explanation

Here,
The given relation is 4\;:\;7\;=\;12\;:\;x
Or, \large\frac{4}{7}=\frac{12}{x}
\rightarrow 4x=84
\rightarrow x=\large\frac{84}{4}
\rightarrow x=21
So that, in the relation the x value is 21.
Ans: x value is 21.

Another method to articulate this particular math query is available:

1. Calculate the value of x in the relation 4 : 7 = 12 : x.
2. Find the unknown value x in the proportion 4 : 7 = 12 : x.
3. If the ratio 4 : 7 is equal to the ratio 12 : x, what is the value of x?