What is the root word of certainty

Certain would be the right answer

The root word is what is left when you take away all the prefixes and the suffixes, the original word which serves as the base word when you are trying to build new words.

So, for example, in the word certainty, when you take away the suffix - ty, which is there to denote a noun derived from an adjective, you get the root word which is certain.