A hydrogen bond is the attractive force between the hydrogen attached to an electronegative atom of one molecule and an electronegative atom of a different molecule. Usually the electronegative atom is oxygen, nitrogen, or fluorine, which has a partial negative charge. The hydrogen then has the partial positive charge.

A hydrogen bond is a partially electrostatic attraction b/w a hydrogen atom which is bound to a more electronegative atom such as N, O and F etc. And also hydrogen bonding occurs only b/w molecules in which H atom bounded to a electronegative atom.

Hydrogen bonding is a special type of intermolecular force that occurs between a hydrogen atom bonded to an electronegative atom (such as oxygen, nitrogen, or fluorine) and an electronegative atom in a nearby molecule. This type of bonding is responsible for many important properties of substances, such as the high boiling point of water and the unique structure of DNA.

To understand why hydrogen is the atom that must be present in hydrogen bonding, let's examine the nature of hydrogen bonds and the characteristics of the other atoms mentioned in the options.

1. Hydrogen (H):
- Hydrogen bonding occurs when a hydrogen atom is bonded to an electronegative atom, such as oxygen or nitrogen.
- Hydrogen has a low electronegativity compared to other atoms, which allows it to form a polar covalent bond with a more electronegative atom.
- The partial positive charge on the hydrogen atom in a polar covalent bond allows it to attract the partial negative charge on another electronegative atom, forming a hydrogen bond.

2. Sodium (Na):
- Sodium is a metallic element and does not form hydrogen bonds.
- It has a low electronegativity and tends to lose its valence electron, forming a cation.
- While sodium can participate in other types of bonding, such as ionic bonding with anions, it cannot form hydrogen bonds.

3. Calcium (Ca):
- Calcium is also a metallic element and does not form hydrogen bonds.
- Like sodium, it has a low electronegativity and tends to lose its valence electrons, forming a cation.
- Calcium can participate in other types of bonding, such as ionic bonding with anions, but it cannot form hydrogen bonds.

4. Sulphur (S):
- Sulphur is a nonmetallic element and has a higher electronegativity compared to hydrogen.
- However, it does not have a hydrogen atom bonded to an electronegative atom, so it cannot form hydrogen bonds.

In summary, only hydrogen atoms bonded to electronegative atoms can form hydrogen bonds. Sodium, calcium, and sulphur do not have the necessary characteristics to form hydrogen bonds. Therefore, the correct answer is option 'A' - hydrogen.