Final answer:
To promote comfort for a client with a small perineal hematoma, applying cold compresses is the correct intervention, as it reduces swelling and pain.
Step-by-step explanation:
The most appropriate nursing intervention to promote comfort for a client who has a small hematoma of the perineal area is applying cold compresses. Cold compresses can help to reduce swelling and numb the pain. Encouraging frequent urination, administering intravenous antibiotics, and administering oxytocin are not suitable interventions for a hematoma in this case.