The sentence indicates that the subject is not sufficiently old to vote. Thus, the answer must mean 'sufficiently old'.
Option A is incorrect. In 'too very old', 'too' and 'very' are both adverbs. Two adverbs cannot be used side by side to modify the same verb. Thus, the phrase itself is grammatically incorrect.
Option B is correct. The phrase 'old enough' means 'sufficiently old'. Thus, it has the required meaning and is the correct answer.
Option C is incorrect. According to the rules of usage used in English Grammar, adjectives used to describe a noun must follow a certain order. According to the rule, adjectives of degree, must come after adjectives of age, size or observation. Thus, 'enough' must come after 'old'. Therefore, 'enough old' is grammatically incorrect.
Option D is incorrect. In 'old for', 'for' is a preposition. Also, 'to' immediately after the blank is a preposition. If 'old for' is used, two prepositions will be placed side by side which will make the sentence grammatically incorrect.