Option : (C)
We know that,
The opposite angles of a parallelogram are equal
Therefore,
∠BCD = ∠BAD = 75°
Now,
In ∆ BCD, we have
∠CDB + ∠DBC + ∠BCD = 180°
(Since, sum of the angles of a triangle is 180°)
∠CDB + 60° + 75° = 180°
∠CDB + 135° = 180°
∠CDB = (180° - 135°)
= 45°