Please help with the question attached in the file. Thank you

Since the condenser(capacitor) plates are oppositely charged, they exert (equal)force on the charged particle in the same direction and hence the total force on the particle is the sum of these two forces. Here the total force is F i.e, force exerted by one plate is F/2. When one of these plates is removed The net force is due to only one remaining plate. Hence the answer will be F/2.