Explanation:

Borax:
Borax is a compound with the chemical formula Na2B4O7·10H2O. It is a hydrated sodium borate that is used in many industrial and household applications.

Anionic part of Borax:
The anionic part of borax is [B4O5(OH)4]2-. It is also known as tetraborate anion.

Hybridization of boron atoms:
The boron atoms in the anionic part of borax are sp3 hybridized. This can be explained by the following steps:

- Boron has 3 valence electrons, so it can form 3 covalent bonds.
- In the tetraborate anion, there are 4 boron atoms.
- Each boron atom is bonded to 3 oxygen atoms and 1 hydroxyl group (-OH).
- The oxygen atoms and the hydroxyl group are electron donors, and the boron atoms are electron acceptors.
- The boron atoms accept electrons from the oxygen atoms and the hydroxyl group to form covalent bonds.
- In order to form these covalent bonds, the boron atoms must hybridize their orbitals.
- The sp3 hybridization of boron atoms allows them to form 4 covalent bonds, including the 3 bonds with oxygen atoms and 1 bond with the hydroxyl group.

Total number of sp3 hybridized boron atoms:
There are a total of 3 boron atoms in the anionic part of borax that are sp3 hybridized. The fourth boron atom is sp2 hybridized because it is bonded to only 2 oxygen atoms and 1 hydroxyl group.

Therefore, the correct answer is '3'.