The correct response to "A serious reduction in the body's water content" is Dehydration.
Dehydration occurs when the body loses more water than it takes in, leading to a deficiency that can affect various bodily functions. It's important to recognize and address dehydration promptly to prevent serious health issues.
Anemia and lactic acid do not directly relate to a reduction in the body's water content, and hydration is the opposite of dehydration.