The correct answer is A) Loss of water and salt due to excessive sweating and inadequate water and salt intake. This is the correct answer because heat exhaustion is caused by a combination of dehydration and electrolyte depletion, specifically loss of water and salt through excessive sweating in a hot environment without adequate replacement. The body loses both water and essential salts like sodium and potassium, leading to the symptoms of heat exhaustion. The other options are incorrect because B) and C) describe situations where the body is losing water and/or salts, but not due to the combination of factors that causes heat exhaustion. Option D) only describes water loss, which is an incomplete explanation for the underlying causes of heat exhaustion.