If ΔPQR and ΔLMN are similar and 3PQ = LM and MN = 9 cm, then QR is equal to:

Answer: Option D

$$\begin{array}{rl}& \frac{PQ}{LM}=\frac{QR}{MN}=\frac{PR}{LN}\\ & \left(△PQR\text{and}△LMN\text{are}\text{similar}\right)\\ & \frac{PQ}{LM}=\frac{QR}{MN}\\ & \frac{1}{3}=\frac{QR}{9}\\ & QR=3\text{cm}\end{array}$$