Which of the following is a symptom of dehydration?

Dehydration occurs when the body loses more fluids than it takes in, leading to a deficit in the body's hydration levels. One of the key symptoms associated with dehydration is the concentration of urine. When the body is dehydrated, the kidneys conserve water and produce less urine, which becomes more concentrated. This results in darker urine and a distinct odor, as the body is trying to retain as much fluid as possible to maintain its functions.

In contrast, increased blood pressure often indicates fluid overload rather than dehydration, fluid retention suggests an excess of fluids in the body typically associated with certain medical conditions, and an increased energy level would not be expected in the context of dehydration, as low fluid levels generally lead to fatigue and decreased physical performance. Therefore, concentration of urine is a clear and direct indicator of dehydration.