Simran is in fourth grade but is still reading at the first grade level. She has normal intelligence and is able to understand concepts and contribute to class discussions. When Simran is given a written quiz, however, she hands in a blank paper. Which of the following best describes Simran's underlying difficulty?

• A. Simran has autism.
• B. Simran has mental retardation.
• C. Simran has a behavioural disorder.
• D. Simran has a learning disability.