I3^- has sp3d hybridisation as it has 3 lone pairs and 2 bond pairs. It is sp3d and three lone pairs of electrons occupy equatorial positions in TBP structure. The shape is almost linear.

Hybridization of I3- ion

- The I3- ion is a polyatomic ion consisting of three iodine atoms bonded together with a negative charge.
- The Lewis structure of I3- ion shows that it has a linear shape with the central iodine atom bonded to two terminal iodine atoms.
- Hybridization is the process of combining atomic orbitals to form hybrid orbitals that are used to describe the bonding in molecules. The hybridization of the central atom in I3- ion can be determined by counting the number of electron groups around it and using the hybridization formula.

Steps to determine hybridization:

- Count the number of electron groups around the central atom. In the case of I3- ion, the central iodine atom is bonded to two terminal iodine atoms, making it a total of three electron groups.
- Determine the hybridization formula using the number of electron groups. For three electron groups, the formula is sp2.
- Apply the hybridization formula to the central atom. In this case, the central iodine atom undergoes sp2 hybridization, which means that its three valence orbitals (one s and two p orbitals) combine to form three sp2 hybrid orbitals.

Therefore, the correct answer is option 'C', sp3 hybridization.