Introduction:
The length of a bond is determined by the distance between the nuclei of the atoms involved. In general, the bond length decreases with increasing bond order. Bond order is the number of chemical bonds between a pair of atoms.

Explanation:
Let's analyze each option and determine the expected bond length for the N to N bond in each molecule:

a) N2H4 (Hydrazine):
- In hydrazine, there are two N to N bonds.
- Each N atom forms a single bond with another N atom.
- Therefore, the bond order is 1.
- The bond length would be longer than a double or triple bond.
- Hence, the N to N bond in N2H4 would be longer than in N2.

b) N2 (Nitrogen gas):
- Nitrogen gas consists of two nitrogen atoms bonded by a triple bond.
- The bond order is 3, which indicates a strong bond.
- A triple bond is shorter and stronger than a single or double bond.
- Therefore, the N to N bond in N2 would be shorter than in N2H4.

c) N2O4 (Dinitrogen tetroxide):
- In dinitrogen tetroxide, there are two nitrogen atoms bonded to each other.
- However, each nitrogen atom is also bonded to two oxygen atoms.
- The nitrogen-oxygen bonds are shorter and stronger than the nitrogen-nitrogen bond.
- Therefore, the N to N bond in N2O4 would be longer than in N2.

d) N2O (Nitrous oxide):
- Nitrous oxide consists of two nitrogen atoms bonded by a double bond.
- The bond order is 2, which is shorter than a triple bond but longer than a single bond.
- Therefore, the N to N bond in N2O would be longer than in N2.

Conclusion:
Based on the analysis above, the N to N bond would be shortest in option b) N2 (Nitrogen gas).