What is the root of the word visibility?

In order to answer this question, you first need to understand what a root word is. A root word is the most basic part of a word which has a meaning on its own and which is used to add prefixes and suffixes to in order to create new words.
In the case of the word visibility, the root word is vis. It comes from Latin verb videre, which means to see. So if you add suffixes to vis, you will create a new word. 
For example, if you add -ibility to it, you will create the word visibility.