What is lanthanide contraction?

Lanthanide contraction is the gradual decrease in atomic and ionic radii of lanthanides from La to Lu with increasing atomic number.

Why does it happen?

Across the lanthanide series, electrons are added to the $4f$ subshell.

• The nuclear charge increases steadily.
• But the added $4f$ electrons do not shield each other effectively.
• Therefore, the effective nuclear charge experienced by outer electrons increases.
• This pulls the electron cloud closer to the nucleus, causing contraction in size.

Evaluate the options

A: Poor shielding of one of $4f$ electron by another in the subshell
This is the standard reason for lanthanide contraction. Since $4f$ electrons shield very poorly, the attraction of the nucleus increases across the series.

B: Effective shielding of one of $4f$ electrons by another in the subshell
Incorrect. If shielding were effective, contraction would be much less.

C: Poorer shielding of $5d$ electrons by $4f$ electrons
This is related to consequences in later transition elements, but it is not the main statement of the cause of lanthanide contraction itself. The accepted main cause is poor mutual shielding among $4f$ electrons.

D: Greater shielding of $5d$ electrons by $4f$ electrons
Incorrect, because $4f$ electrons shield poorly, not greatly.

Conclusion

The main cause of lanthanide contraction is: $\boxed{\text{Poor shielding of one }4f\text{ electron by another}}$

So the correct option is A.