Identify the concept

The question asks which method is used to obtain boron and silicon of very high purity.

These are semiconductor elements, and for semiconductor applications, extremely high purity is required.

Recall the refining methods

• Liquation: Used for metals with low melting point such as tin, lead, bismuth.
• Electrolytic refining: Common for metals like copper, silver, gold, zinc, etc.
• Zone refining: विशेष method used for obtaining ultra-pure semiconductors like silicon, germanium, boron.
• Vapour phase refining: Used when a volatile compound can be formed and decomposed, e.g. Mond process for Ni, Van Arkel process for Ti and Zr.

Why zone refining works

In zone refining, a narrow molten zone is passed through the solid. Impurities are more soluble in the molten phase than in the solid phase, so they move with the molten zone and get concentrated at one end. Repeating this process gives very high purity.

This method is especially suitable for silicon and boron.

Evaluate the options

• A: Liquation $\rightarrow$ Not suitable.
• B: Electrolytic refining $\rightarrow$ Not the standard method for ultra-pure B and Si.
• C: Zone refining $\rightarrow$ Correct.
• D: Vapour phase refining $\rightarrow$ Not the correct method here.

Final answer

Therefore, the correct option is: $\boxed{\text{C: Zone refining}}$

Comparison with stored answer

Stored correct answer = C.

My derived answer is also C, so they agree.