Final answer:
The best indication that a 15-month-old child is experiencing pain after surgery is increased irritability, which reflects the child's discomfort when they cannot verbally express it.
Step-by-step explanation:
The best indication that a 15-month-old child is experiencing pain after surgery to remove a Wilms' tumor would be increased irritability. Pain in children, especially those too young to effectively communicate, is often manifested through changes in behavior. Increased irritability may reflect the child's discomfort, which they cannot express in words. Unlike options such as increased appetite or increased playfulness, irritability is a common sign of distress in children. Changes in vital signs such as heart rate can also indicate pain, but a decrease in heart rate is not typically a response to pain; rather, an increase might be expected. Therefore, the correct answer is d. Increased irritability.