Ultra pure silicon can be obtained by

Zone refining

The correct answer is option 4. Zone refining.

Ultra-pure silicon, which is often used in the semiconductor industry, can be obtained by various purification methods. Let us analyze each option:

1. Distillation: Distillation is a method commonly used for purifying liquids based on differences in boiling points. However, for solids like silicon, distillation is not a practical method for purification.

2. Electrolytic refining: Electrolytic refining involves passing an electric current through a molten or dissolved compound to remove impurities. While this method is used for refining metals, it is not typically employed for silicon purification.

3. Vapour phase refining: Vapour phase refining, also known as chemical vapour deposition (CVD), is a technique used to deposit thin films of materials onto substrates. It is not specifically used for purifying silicon to ultra-pure levels.

4. Zone refining: Zone refining is a highly effective method for purifying silicon to ultra-pure levels. In this process, a rod of impure silicon is gradually passed through a high-frequency induction coil, creating a molten zone that travels along the rod. As the molten zone moves, impurities are segregated to one end of the rod, resulting in ultra-pure silicon at the other end. Zone refining is widely used in the semiconductor industry for producing ultra-pure silicon.

Therefore, the correct answer is: 4. Zone refining