(d) 1 cm
In △OAB and △OPQ, AB ∥ PQ, so △OAB ~ △OPQ (AA similarity).
$$\frac{OP}{OB} = \frac{PQ}{AB} \implies \frac{OP}{3} = \frac{2}{6} = \frac{1}{3} \implies OP = 1 \text{ cm}$$
Since AB ∥ PQ, the two triangles OAB and OPQ are equiangular (AA criterion). Use the ratio of corresponding sides: PQ/AB = 2/6 = 1/3. Apply this ratio to OB = 3 cm to get OP = 1 cm. A common mistake is using OB/OP = AB/PQ instead of the correct correspondence OP/OB = PQ/AB.