Final answer:
The best indicator of dehydration in the described scenario is increased drowsiness, mild confusion, and concentrated urine, which are direct effects of the body's response to insufficient fluid intake.
Step-by-step explanation:
The assessment finding that would best indicate the presence of dehydration in an elderly nursing home resident who has not been eating or drinking as much as usual is c. Increased drowsiness, mild confusion, and concentrated urine. These symptoms reflect the physical effects of dehydration on the body, such as exhaustion, collapse, and an increase in the concentration of urine due to the body attempting to conserve water. Dehydration can occur from various causes like vomiting or diarrhea, excessive sweating, and not consuming enough fluids, leading to a host of symptoms including dryness of skin, increased pulse rate, reduced cardiac output, rise in body temperature, and mental changes such as confusion.