The correct answer is C. Two-thirds is a fraction
Explanation:
The hyphen is a punctuation mark usually used to link two or more elements in a sentence, hyphened words usually act as a modifier or adjective before a noun. According to grammar, if a sentence includes a fraction, the fraction should be hyphened as both parts of the fraction are linked and this is shown through the use of hyphen that clarifies the relationship between the two parts of the fraction "two" and "thirds". Additionally, fractions are hyphened especially is the fraction acts as a modifier in the sentence. Considering this, the hyphen is "two-thirds" is explained because this is a fraction and a hyphen is required to let the reader know the relation between the two words and thus, the reader can understand they are both parts of a fraction and not just separated numbers.