The correct answer is the thumb of the right hand.
Explanation:
The Force will follow Lorentz's law, a vectorial product: F= q*v x B, where:
q is the charge of the particle.
v is its velocity.
x indicates the vector product.
B is the magnetic field.
Being a vector product F the plane formed by v and B will be perpendicular. To determine the sense we will apply the variant of the rule of the right hand. The fingers of the right hand without the thumb run from vector v to vector B, so in this case, the force will have a sense upwards. It must be said that the rule always indicates the direction of the positive charge if the charge is negative the direction of F is inverse (downwards).
