Choice Bhoice D is the correct answer.

1. Start with the equation: $3x - 15 = 0$
2. Add 15 to both sides: $3x = 0 + 15$
3. Simplify: $3x = 15$
4. Divide both sides by 3: $x = \frac{15}{3}$
5. Simplify: $x = 5$
6. Verify: $3(5) - 15 = 15 - 15 = 0$ ✓

Choice Bhoice C is incorrect because this might result from thinking $3(-5) = -15$, but $-15 - 15 = -30$, not 0.

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

Choice Bhoice C is incorrect because this results from multiplying 15 by 3: $45$.