The correct option is C long
The given sentence requires the most suitable form of adjective. This sentence is a general statement. It isn't a comparative statement. It just states a fact. We use the positive degree of adjectives for general statements. Hence, we can say - 'Discipline is essential to live long.' Thus, the correct answer is C - Long. Option A - 'Longer' is incorrect as we aren't comparing anything here, we can't use the comparative degree of adjective. Option B is incorrect, as 'longest' is a superlative degree of adjective and shouldn't be used for general statements. Option D is incorrect, as option C is the answer.