Nathan frequently uses his mobile phone to send text messages, yet he cannot immediately recall what number is on the button with the letter "P." This memory difficulty most clearly illustrates which of the theories of forgetting? Why?

encoding failure

Explanation:

In psychology, encoding failure can be described as the event when the brain fails to establish a memory link. This phenomenon is common in many people. In psychology, encoding means the ability of a brain to store and refresh a memory. Hence, encoding failure means the inability to refresh a memory. In the scenario discusses in the question, Nathan's inability to remember the number with the letter P can be attributed to encoding failure.