Answer:
c) E/2
Step-by-step explanation:
The relationship between magnitude of electric field (E), distance between the plates (d) and voltage across the plates (V), for a uniform electric field, is given by:

Re-arranging it, we can write it as

in the problem, the potential across the plates is kept constant: V' = V.
However, the distance between the plates is doubled: d' = 2d
Therefore, we can calculate the new magnitude of the electric field:

So the correct answer is
c)E/2