The field between two charged parallel plates is kept constant. if the two plates are brought closer together, the potential difference between the two plates either a) decrease b) does not change c) increase?

Since the electric field between the plates is constant

so we can say the relation between potential difference and electric field is given by

here from above relation we can say that is electric field is kept constant then potential difference is directly proportional to the distance between the plates

so here we will say that if the distance between the plates is decreased then the potential difference between the plates will also decreases

So correct answer will be

a) decrease