Examine Assertion (A)

Assertion: The first ionization enthalpy of $3d$ series elements is more than that of group 2 metals.

• The $3d$ series starts from Scandium ($\mathrm{Sc}$) and continues to Zinc ($\mathrm{Zn}$).
• Group 2 metals in the corresponding periods are mainly:
  • $\mathrm{Mg}$ in period 3
  • $\mathrm{Ca}$ in period 4
• Across a period, first ionization enthalpy generally increases because effective nuclear charge increases.
• The $3d$ elements generally have higher first ionization enthalpy than alkali metals, but not necessarily more than group 2 metals in a blanket sense.

In fact, comparing with $\mathrm{Ca}$:

• $\mathrm{Ca}$ has first ionization enthalpy around $590\,\text{kJ mol}^{-1}$
• $\mathrm{Sc}$ is around $633\,\text{kJ mol}^{-1}$, but several $3d$ values fluctuate and the statement as a general periodic claim is not taken as a correct reasoned rule in this form.

More importantly, the standard periodic trend is that transition elements do not uniformly have first ionization enthalpy greater than group 2 metals as a general assertion. Hence Assertion (A) is taken as false.

Examine Reason (R)

Reason: In $3d$ series of elements successive filling of d-orbitals takes place.

• This is correct.
• In the first transition series, electrons are progressively added to the $3d$ subshell.

Therefore, Reason (R) is true.

Match with options

• (A) is false
• (R) is true

Hence, the correct option is:

$\boxed{\text{D}}$

Compare with stored correct answer

Stored correct answer: $\text{D}$

Our derived answer matches the stored answer.