Zone refining is based on the principle that

The correct answer is Option (2) → Impurities are more soluble in molten metal than in solid metal

In zone refining the impurities are more soluble in the melt than in the solid metal, so a moving molten zone carries them to one end of the rod while pure metal crystallises behind it.

Note: This question is from the pre-rationalised NCERT book (2022-23). The topic is not there in the latest NCERT book (2026-27), and it is also out of the CUET 2026 syllabus.

NCERT (Pre-rationalised edition, 2022-23):
Zone Refining