Correct option (a) Both A and R are true and R is the correct explanation of A
Explanation:
According to Aufbau's principle, the unfilled shell with lowest (n + l) value will be filled first. Therefore 4s orbital is filled first than 3d orbital.
For 4s - orbital, (n + l) = 4 + 0 = 4
For 3d - orbital, (n + l) = 3+2 = 5