Which term describes a person on the far left of the political spectrum?

a. reactionary

b. conservative

c. moderate

d. radical

I'd say "radical". It is often used to describe liberals, but I have heard "radical conservative". Still, if you hear just the term "radical", it often means a far-left person.

A "radical" would be the best option in terms of a description of a person on the far left of the political spectrum, but this is used for people on the right as well.