To answer this question, we can proceed as follows:
As we can see from the above figure, the triangles PQR and MQN are similar triangles. If they are similar, then their corresponding sides are in proportion. Then, we have:

Then, we need to solve the equation for MN, then we have:
![(15)/(MN)=5\Rightarrow(MN)/(15)=(1)/(5)\Rightarrow MN=(15)/(5)\Rightarrow MN=3\operatorname{cm}]()
Therefore, the value for the side MN is 3 cm (option b).