Concept:"One man, one vote" describes the democratic principle used in co-operative societies.Explanation:In a co-operative society, every member has one vote regardless of the amount of capital contributed.This ensures equal control and democratic decision-making among members.Other business units such as sole proprietorship, partnership, or public company do not follow this voting principle.Therefore, the business unit that applies "one man, one vote" is a co-operative society.Answer:D. co-operative society