Option B: 27 cm
Since △PQR ~ △ABC, the ratio of their perimeters equals the ratio of corresponding sides.
$$\frac{\text{Perimeter of } \triangle PQR}{\text{Perimeter of } \triangle ABC} = \frac{PQ}{AB} = \frac{6}{8} = \frac{3}{4}$$
$$\text{Perimeter of } \triangle PQR = \frac{3}{4} \times 36 = 27 \text{ cm}$$
When two triangles are similar, the ratio of their perimeters equals the ratio of their corresponding sides (scale factor). Here the scale factor is PQ/AB = 6/8 = 3/4, so multiply the known perimeter (36 cm) by this ratio. Students often mistakenly square the ratio — that is only for areas, not perimeters.