Hydrogen gas is the key element used in making banaspati ghee through a process called hydrogenation. Banaspati ghee is a popular cooking fat in many households, especially in South Asia. It is made by converting vegetable oils, which are usually liquid at room temperature, into a semi-solid form. This transformation happens through a chemical process known as hydrogenation, and the main gas involved in this is hydrogen.

During hydrogenation, hydrogen gas is passed through the vegetable oil in the presence of a metal catalyst like nickel. This process adds hydrogen atoms to the unsaturated fat molecules in the oil, making them more saturated. As a result, the oil becomes thicker and gains a texture similar to that of traditional ghee.

This method not only makes the product shelf-stable and solid at room temperature, but it also enhances the taste and appearance. However, excessive consumption of hydrogenated fats is known to be unhealthy because it can increase trans fat content, which is linked to heart diseases.

Let’s also look at the other options:
- **Oxygen** is not used, as it can oxidize the oil and spoil it.
- **Nitrogen** is used in food packaging but not in the ghee manufacturing process.
- **Helium** has no role in hydrogenation.

So, the correct answer is **Hydrogen**, which plays a crucial role in converting liquid oils into the creamy, semi-solid form we know as banaspati ghee. Understanding the science behind common food items helps us make better dietary and health choices.