Answer
F = [ML$T^{-2}$]
Work Step by Step
Basically,
a $\propto \dfrac{F}{m}$. So
a = k $\dfrac{F}{m}$, where k is a dimensionless constant.
Dimension of acceleration is [L$T^{-2}$].
Dimension of mass is [M].
Writing the above equation dimensionally gives
[L$T^{-2}$] = F [$M^{-1}$].
Both sides have the same dimensions, and, therefore,
F = [ML$T^{-2}$].
