Answer
Charge a is positive.
Charge b has no charge.
Charge c is negative.
Work Step by Step
The magnetic field points into the page.
Charge a feels an initial force up the page. The right-hand rule tells us that charge a is positive.
Charge b feels no force, though it is moving perpendicular to a magnetic field, so it carries no charge.
Charge c feels an initial force down the page. The right-hand rule tells us that charge c is negative.