(d) $\tau_{1} = RC\left( \frac{1 \times 2}{1 + 2} \right) \times \left( \frac{2 \times 4}{2 + 4} \right) = \frac{2}{3} \times \frac{8}{6} = 8/9\ \mu\ s$
$\tau_{2} = RC = (1 + 2)(2 + 4) = 3 \times 6 = 18\ \mu\ s$
$\tau_{3} = \left( \frac{1 \times 2}{1 + 2} \right)(2 + 4) = \frac{2}{3} \times 6 = 4\ \mu\ s$