If projected with identical velocity perpendicular to a magnetic...

The magnetic force on a charged particle moving perpendicular to a magnetic field is given by $F = qvB$.

Since the velocity $v$ and magnetic field $B$ are identical for all particles, the magnitude of the magnetic force is directly proportional to the magnitude of the charge $q$ of the particle.

The magnitudes of the charges for the given particles are:
Proton: $e$
$\text{He}^+$: $e$
Electron: $e$
$\text{Li}^{++}$: $2e$

Since $\text{Li}^{++}$ has the maximum charge magnitude, it will experience the maximum magnetic force.