Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). 

Assertion: Ionisation enthalpy increases along each series of the transition elements from left to right. However, small variations occur.
Reason: There is corresponding increase in nuclear charge which accompanies the filling of electrons in the inner d-orbitals.

In the light of the above statements, choose the most appropriate answer from the options given below :