Step 1: Frame minimal brain dysfunction as a disorder of a child with broadly preserved intelligence but uneven, immature control of behaviour, attention and learning.
Step 2: Its standout problems are specific learning disabilities - trouble blending auditory and visual functions for language and difficulty reading, writing and doing mathematics.
Step 3: The textbook example of such a specific learning disability quoted under this syndrome is dyslexia, a focused reading deficit rather than a global one.
Step 4: The other options describe whole-brain intellectual impairment (mental subnormality, Down's syndrome) or a behaviour-attention disorder (ADHD), which do not capture the specific-learning-disability core, so dyslexia is selected.
\[\boxed{\text{Dyslexia}}\]