A learning disability that affects the acquisition of arithmetical skill is:

Specific learning disabilities (SLDs) are neurodevelopmental conditions in which a person of normal intelligence has significant, persistent difficulty in one particular academic skill domain -- reading, written expression, or arithmetic -- while other abilities are largely unaffected. Each named disorder is defined by the single skill domain it disrupts, not by the learner's overall intelligence.

The stem asks for the disability that specifically affects the acquisition of arithmetical skill -- that is, difficulty with numbers, counting, and calculation. That numeracy-specific impairment is Dyscalculia -- a term combining the Greek prefix "dys-" (difficulty) with the Latin root "calculare" (to count/reckon).

• Dyscalculia -- impairs the arithmetic/number-skill domain (counting, calculation, number sense); matches the stem.

• Dysgraphia -- impairs the written-expression/handwriting domain, not arithmetic.

• Dyslexia -- impairs the reading domain (word decoding and fluency), not arithmetic.

• Dysmagnisia -- not a recognized term for any specific learning disability in special-education or clinical literature.

Because the impairment described is specific to arithmetic skill, the correct classification is Dyscalculia.