The tracks of three charged particles in a uniform electrostatic field are shown in the figure. Which particle has the highest charge to mass ratio?

Particles $A$ and $B$ have negative charges because they are being deflected towards the positive plate of the electrostatic field. Particle $C$ has positive charge because it is being deflected towards the negative plate. 

Deflection of charged particle in time $t$ in $y-$direction

$h=0\times t+\frac{1}{2}a{t}^2=\frac{1}{2}\frac{qE}{m}{t}^2 i.e., h\infty q/m$

As the particle $C$ suffers maximum deflection in $y-$direction, so it has highest charge to mass ($q/m$) ratio.