The correct answer is: two-thirds
Hyphens are used to link words and parts of words, they show the reader that two or more elements in a sentence are related. Following the grammatical rules, they can be used when a fraction is spelled, except when a or an is used to introduce it.
For example:
I gave you one-third of my cake.
I gace you a third of my cake.
In a or an is not used so a hyphen can be used.
