Choice Choice D is the correct answer.

1. Start with the equation: $3x - 12 = 0$
2. Add 12 to both sides: $3x = 0 + 12$
3. Simplify: $3x = 12$
4. Divide both sides by 3: $x = \frac{12}{3}$
5. Simplify: $x = 4$
6. Verify: $3(4) - 12 = 12 - 12 = 0$ ✓

Choice Choice C is incorrect because this might result from thinking $3(-4) = -12$, but $-12 - 12 = -24$, not 0.

Choice Choice A is incorrect because this is the value of $3x$. You must divide by 3.

Choice Choice B is incorrect because this results from multiplying 12 by 3: $36$.