Final answer:
The best meaning for the bolded word is 'the source of something'.
Step-by-step explanation:
The best meaning for the bolded word in the given sentence is The source of something. When people say that money is the root of all evil, they mean that it is the source or cause of all evil. In this context, 'the root' is used metaphorically to refer to the source or origin. It does not refer to digging, tooth base, or the underground part of a plant.
Learn more about Meaning of a word in context