Step 1: Minimal brain dysfunction (minimal dysfunction syndrome) is a neurodevelopmental disorder marked by immaturity in the control of activity, emotions and behaviour, despite near-normal intelligence.
Step 2: Its core features are specific learning disabilities in the communication skills needed for reading, writing and mathematics, along with difficulty maintaining attention and concentration.
Step 3: A defining specific learning disability within this syndrome is dyslexia (a reading disability), so dyslexia is the classic clinical feature listed for minimal dysfunction syndrome. Hence option (a) is the answer.
Step 4: The distractors are less correct: although ADHD overlaps with attention features, mental subnormality and Down's syndrome are global intellectual disabilities, not the specific learning-disability picture; the answer as keyed is dyslexia.