Answer
The more massive particle is positively charged.
Work Step by Step
The force on the particle due to the magnetic field provides the centripetal force to keep the particle moving in a circle.
We can find an expression for the radius of motion:
$F_B = qv\times B = \frac{mv^2}{r}$
$r = \frac{mv}{qB}$
If we assume that the two particles have the same speed, the more massive particle has a larger radius of motion.
The path with the larger radius corresponds to the more massive particle.
Note that the direction of the force on the particle is given by $v \times B$
By the right hand rule, the positively charged particle is moving counter-clockwise, since the magnetic field is directed into the plane of the page.
The particle moving counter-clockwise has a larger radius of motion.
Therefore, the more massive particle is positively charged.
