"The French" is a plural noun which refers to the people of France collectively. In case of collective plural nouns, plural form of verb is used to modify the noun. Also, the sentence uses the main verb "reputed" which indicates generally supposed to be such. In such cases the simple present tense is considered correct. Hence, the plural auxiliary verb "are" is the correct answer. Option C is correct.
Option A is incorrect as "is" is a singular verb.
Option B is incorrect as "was" is used for past tense.
Option D is incorrect as  "will be" is used for future tense.