During dehydration, the substance that is usually lost by the body is sodium chloride (option 2). Dehydration occurs when the body loses more fluids than it takes in. Sodium chloride, also known as salt, is an essential electrolyte that helps to maintain the balance of fluids in the body. It plays a crucial role in maintaining proper hydration levels, nerve function, and muscle contraction. When the body loses water through sweating, urination, or other means, sodium chloride is also lost along with it. This loss of sodium chloride can disrupt the balance of electrolytes in the body, leading to dehydration. Therefore, it is important to replenish fluids and electrolytes, including sodium chloride, to prevent and treat dehydration.