Figure shows tracks of three charged particles in a uniform electrostatic field. Give the signs of the three charges. Which particle has the highest charge to mass ratio?

Since particles (1) and (2) are deflecting towards the positively charged plate both of these particles are negatively charged. Particle (3) is positively charged because it is getting deflected towards negatively charged plate.

Deflection experienced by a particle is directly proportional to charge/mass ratio.
Therefore, since particle 3 experiences maximum deflection it has the maximum charge to mass ratio.