Final answer:
(1) Administering an oral rehydration solution is the most effective activity to manage severe diarrhea in an infant, as it helps replace lost fluids and electrolytes.
Step-by-step explanation:
The most effective activity to include in the plan of care for an infant experiencing severe diarrhea would be administering oral rehydration solution. Oral rehydration solutions are crucial for correcting dehydration in patients with gastroenteritis, as they comprise a mixture of water, salts, and sugar optimized to replace lost fluids and electrolytes.
Feeding solid foods or applying cold compresses may not be as beneficial during acute gastrointestinal illness and severe dehydration, and a warm bath would not address the primary concern of dehydration.