Final answer:
The mother needs further instructions because scratching underneath the cast can lead to skin irritation and damage the healing process.
Step-by-step explanation:
To properly care for a plaster cast, it is important to follow certain instructions. The statement that indicates the need for further instructions is option c) 'I will encourage my child to scratch underneath the cast with a pencil.' Scratching underneath the cast can lead to skin irritation and potentially damage the healing process. The other statements are correct: keeping the cast clean and dry, using a hairdryer on the cool setting if it gets wet, and monitoring for signs of skin irritation or swelling.