Final answer:
A nurse should report a bulging fontanelle to the provider as this may indicate a complication with the VP shunt, such as underdraining.
Step-by-step explanation:
Among the options provided, the nurse should report a bulging fontanelle to the provider. This is a sign that may indicate increased intracranial pressure, which could suggest the VP shunt is not functioning properly or is having complications such as underdraining. On the other hand, decreased irritability, increased feeding tolerance, and improved head circumference are positive signs indicating improvement postoperatively and potentially successful VP shunt function.