Step 1: Evaluate Assertion (A): separation of Zr and Hf is difficult.
Zr (Z=40) and Hf (Z=72) are both in Group 4. Their separation is indeed very difficult even with modern techniques. Assertion (A) is TRUE.
Step 2: Evaluate Reason (R): same group causes difficult separation.
It is TRUE that they are in the same group. However, many same-group pairs (like Ti and Zr, or Na and K) are easy to separate. Being in the same group alone does not adequately explain why Zr and Hf are uniquely difficult to separate. Reason (R) is TRUE but NOT the correct/complete explanation.
Step 3: Find the actual reason: lanthanoid contraction.
Between Zr (period 5) and Hf (period 6), the 14 lanthanide elements occupy period 6. Due to poor shielding by 4f electrons, the effective nuclear charge on Hf's outer electrons is greatly increased across the lanthanides. This causes Hf to have almost the same atomic radius as Zr despite being one period lower.
Step 4: Compare radii and properties of Zr and Hf.
$r_{Zr} = 160$ pm; $r_{Hf} = 159$ pm. These are virtually identical! Because of the same radii, Zr and Hf have almost identical chemical properties (same charge, same size, same chemistry), making their separation extremely difficult.
Step 5: Assess the options.
Both A and R are TRUE statements. However, R (same group) is NOT the correct explanation of A. The correct explanation involves lanthanoid contraction making Zr and Hf almost identical in size and properties.
Step 6: State the final answer.
Both A and R are true, but R is NOT the correct explanation of A. \[ \boxed{\text{Both A and R true, but R is not the correct explanation of A}} \]