The correct answer is Option (3) → Lanthanoid contraction
Zr and Hf occur together in nature and are difficult to separate because of lanthanoid contraction.
Due to the poor shielding effect of 4f electrons across the lanthanoid series, there is a steady decrease in atomic size known as lanthanoid contraction. As a result, zirconium and hafnium end up having almost identical atomic and ionic radii and very similar chemical properties. Because of this close similarity, they occur together in nature and are very difficult to separate.
Option-wise check
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Filling of same d orbital — Incorrect
They are in the same group but their similarity is not due to identical d-orbital filling.
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Actinoid contraction — Incorrect
This applies to actinoids, not to Zr and Hf.
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Lanthanoid contraction — Correct
Causes nearly identical sizes and chemistry.
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Both belong to same transition series of d-block elements — Incorrect
Zr is in the 4d series and Hf is in the 5d series.
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