Chemical symbol for nitrogen gas
The chemical symbol for nitrogen gas is N2.

Explanation:
- Nitrogen gas is a diatomic molecule, meaning it consists of two nitrogen atoms bonded together.
- In chemical notation, the symbol for nitrogen is 'N', and the subscript '2' indicates that there are two nitrogen atoms in the molecule.
- The chemical formula for nitrogen gas is therefore N2.

Importance of chemical symbols:
- Chemical symbols are abbreviations used to represent elements and compounds in chemistry.
- They provide a concise and standardized way to identify and communicate about different substances.
- Chemical symbols are based on the element's name or Latin name, and are usually composed of one or two letters.

Other options:
a) N2: This is the correct answer. It represents nitrogen gas, which is a diatomic molecule consisting of two nitrogen atoms bonded together.
b) N: This represents a single nitrogen atom, not nitrogen gas. It is the chemical symbol for nitrogen as an element.
c) N3: This is not the correct answer. It represents a hypothetical molecule with three nitrogen atoms bonded together, which does not exist.
d) Ni: This represents the element nickel, not nitrogen.

Conclusion:
The correct chemical symbol for nitrogen gas is N2, as it represents the diatomic molecule consisting of two nitrogen atoms bonded together.