The correct answer is C) excessive loss of water and salt from the body. Heat exhaustion is caused by the body's inability to regulate its temperature, leading to excessive loss of water and salt through sweating. This can happen when working or exercising in hot environments, especially if the person is not properly hydrated and replacing the fluids and electrolytes lost through sweating. The other options are incorrect - option A refers to water intoxication, option B describes heat stroke, and option D is the opposite of what causes heat exhaustion.