To solve the equation \( \frac{x}{12} = 6 \) for \( x \), you should isolate \( x \) by eliminating the fraction. The correct operation to do on both sides of the equation is to multiply by 12.
So the answer is: D: multiply by 12
This will give you:
\[ x = 6 \times 12 \]
Calculating that, you find \( x = 72 \).