To find the value for \( x \) in the equation \( 21x = 630 \), we need to isolate \( x \). This can be done by dividing both sides of the equation by 21:
\[ x = \frac{630}{21} \]
Now, let's perform the division:
\[ 630 \div 21 = 30 \]
So, the value of \( x \) that makes the sentence true is:
\[ \boxed{30} \]
Thus, the correct answer is C. 30.