a
(a) Let $n = k{\rho ^a}{a^b}{T^c}$ where $[\rho ] = [M{L^{ - 3}}],\;[a] = [L]$ and $[T] = [M{T^{ - 2}}]$
Comparing both sides, we get
$a = \frac{1}{2},\,b = \frac{3}{2}$ and $c = \frac{{ - 1}}{2}$
$\eta = \frac{{k{\rho ^{1/2}}{a^{3/2}}}}{{\sqrt T }}$