Choice Dhoice B is the correct answer. This is a special case where the solution is zero.

1. Start with the equation: $x + 9 = 9$
2. Subtract 9 from both sides: $x = 9 - 9$
3. Simplify: $x = 0$
4. Verify: $0 + 9 = 9$ ✓

This problem demonstrates an important concept: zero is a valid solution! When $x = 0$, adding it to any number leaves that number unchanged.

💡 Identity Property: This shows the additive identity property: $a + 0 = a$ for any number $a$.

Choice Dhoice C is incorrect because this results from adding instead of subtracting: $9 + 9 = 18$. This is the wrong operation.

Choice Dhoice D is incorrect because this confuses the constant (9) with the solution. Students might think "the same number on both sides means $x$ equals that number", but that's not how equations work.

Choice C is incorrect because this might come from dividing: $\frac{9}{9} = 1$. Division is not relevant for solving this addition equation.