a
\(\text { Density }=\left[{F}^{{a}} {L}^{{b}} {T}^{{c}}\right]\)

\({\left[{ML}^{-3}\right]=\left[{M}^{{a}} {L}^{{a}} {T}^{-2 {a}} {L}^{{b}} {T}^{{c}}\right]}\)

\({\left[{M}^{1} {L}^{-3}\right]=\left[{M}^{{a}} {L}^{{a}+{b}} {T}^{-2 {a}+c}\right]}\)

\({a}=1 ; \quad {a}+{b}=-3 \quad ; \quad-2 {a}+{c}=0\)

\(\quad 1+{b}=-3 \quad {c}=2 {a}\)

\({b}=-4 \quad {c}=2\)

So, density \(=\left[{F}^{1} {L}^{-4} {T}^{2}\right]\)