It's a charged particle in a magnetic field and electric field. If the particle is passing through undeflected this means that the magnetic field force and electric field force are equal in magnitude but opposite in direction

So basically we can equate them: F=qE from an electric field and F=qvB from a magnetic field, giving:

qE = qvB

E = vB

We know that E=200 V/m, v = 250 m/s and so can solve for B

200 = 250 B

therefore B = 0.8 T

Now the polarity of M relative to N all depends on which way the forces are going (remember we want them to be opposite in direction so it moves through undeflected)

If we use the right hand palm rule as it is a positive particle we see that the magnetic force is pointing upwards

This means the electric field force must point downwards to cancel out the magnetic field force that points upwards. To have this happen the bottom plate needs to be negative so it attracts the positive particle (opposite charges attract). This means the top plate is therefore positive

So the answer would A