Final answer:
The nurse should use the Wong-Baker Faces scale to assess pain in a mentally challenged adult client, as it provides a visual way to quantify pain through facial expressions, from no pain to worst pain.
Step-by-step explanation:
The appropriate scale for a nurse to use when assessing pain in a mentally challenged, adult client who has had a fall is the Wong-Baker Faces scale. This scale provides a visual representation of pain levels through faces showing different expressions, ranging from smiling (no pain) to crying (worst pain), which is beneficial in cases where verbal communication may be limited. It is a type of pain assessment tool that allows individuals to rate their pain on a scale, usually from 0-10, which can be helpful in providing a quantifiable measure of the pain the person is experiencing.