Good afternoon.

Here at c and d potential is same.

How sir.

Explain in mathematically.

Dear student,

Instead of understanding this mathematically, you can understand this using symmetry.
The circuit is symmetric about the axis passing through points a and b. Thus the way potential drops in the upper half is same as the way it drops in lower half across the capacitors. Since there is symmtery(since the capacitors in upper and lower halves have same value), the potential drop across the two capacitors in left(upper and lower ones) will have same potential drop across them. And so will the potentials at points c and d be equal.

Hope it helps. Regards.