d
(d) \([n]\) = Number of particles crossing a unit area in unit time = \([{L^{ - 2}}{T^{ - 1}}] \)

\(\left[ {{n_2}} \right] = \left[ {{n_1}} \right] \) = number of particles per unit volume \(= [L-3]\)

\( [{x_2}] = [{x_1}]\)= positions  \(D = \frac{{[n]\;\left[ {{x_2} - {x_1}} \right]}}{{\left[ {{n_2} - {n_1}} \right]}} = \frac{{\left[ {{L^{ - 2}}{T^{ - 1}}} \right] \times [L]}}{{[{L^{ - 3}}]}} \) \(=\left[ {{L^2}{T^{ - 1}}} \right]\)