What indicates a complete understanding of measures to relieve itching under a cast?

Using a hair dryer on the low setting is a valid method to help relieve itching under a cast. This technique can provide a gentle stream of air that cools the skin and alleviates the sensation of itchiness without introducing any moisture or causing skin irritation, which can occur with other methods.

The other choices suggest measures that could potentially worsen the situation. Scratching outside the cast may provide temporary relief but does not address the underlying cause of the itch and could lead to longer-term skin issues or tangling with the cast material. Applying lotion directly into the cast isn't advisable as it introduces moisture, which can lead to skin maceration or infection while also diminishing the integrity of the cast. Removing the cast due to itching is not a safe option unless directed by a healthcare provider, as it could impede the healing process and expose the skin to further complications.