(d) \(\frac{mn(n-1)(m-1)}{4}\)
A parallelogram is formed by choosing two straight lines from the set of m parallel lines and choosing two straight lines from the set of n parallel lines.
∴ Number of parallelograms formed = mC2 × nC2
= \(\frac{m(m-1)}{2}\times\frac{n(n-1)}{2}\) = \(\frac{mn(n-1)(m-1)}{4}\)