SSC MTS 2 Aug 2019 Shift 2 Paper

The correct answer is option:3) Kindest.
The sentence refers to one teacher and her attribute among the others. Her kindness is the highest among all the teachers.
A superlative degree is used to choose one out of a group or many.

• Base form: kind
• Comparative: kinder
• Superlative: kindest

Thus kindest is the most suitable option.