Hybridization in (C6H5)3C (on negative carbon) is sp2.

Explanation:
1. Determine the central atom: In this case, it is the carbon atom with the negative charge.
2. Count the total number of valence electrons on the central atom: Carbon has 4 valence electrons, and there are 3 phenyl groups attached to it, each with 6 valence electrons. The negative charge adds an extra electron, so the total number of valence electrons is 4 + 3(6) + 1 = 23.
3. Determine the number of electron pairs around the central atom: In this case, there are 4 electron pairs around the central atom (3 bonds and 1 lone pair).
4. Determine the hybridization: The number of electron pairs around the central atom suggests sp3 hybridization, but the phenyl groups are bulky and prevent the lone pair from participating in bonding. Therefore, the lone pair stays in an unhybridized p orbital and the hybridization becomes sp2.
5. Draw the Lewis structure: The carbon atom is bonded to three phenyl groups and has a lone pair in an unhybridized p orbital.

Overall, the hybridization in (C6H5)3C (on negative carbon) is sp2 due to the presence of bulky phenyl groups that prevent the lone pair from participating in bonding.