Choice Ahoice C is the correct answer.

1. Define: $W=w, L=w+3$.
2. Perimeter: $2(w + w + 3) = 30$
3. Simplify: $2(2w + 3) = 30 \rightarrow 4w + 6 = 30$
4. Subtract 6: $4w = 24 \rightarrow w = 6$.
5. Length: $6 + 3 = 9$.

Choice Ahoice D is incorrect because this is the width.

Choice Ahoice B is incorrect because calculation error.

Choice Ahoice C is incorrect because calculation error.