Final answer:
Signs of severe dehydration include hypotension, tachycardia, and oliguria, which indicate a decrease in blood volume and the body's attempt to conserve water.
Step-by-step explanation:
The most indicative signs of dehydration in an elderly man who has experienced several days of nausea, vomiting, and diarrhea would be B. Hypotension, tachycardia, oliguria. Dehydration can decrease the blood volume leading to hypotension (low blood pressure), and the body may compensate by increasing the heart rate, resulting in tachycardia (fast heart rate). Oliguria, or reduced urine output, reflects the body's attempt to conserve water. These symptoms are alarming and suggest a serious hydration problem that requires immediate medical attention.