What condition is most commonly associated with an extra copy of the twenty-first chromosome?

The presence of an extra copy of the twenty-first chromosome is known as Down syndrome, which is indeed most commonly associated with cognitive impairment and distinct facial characteristics. Individuals with Down syndrome often exhibit a range of cognitive abilities, but many experience some level of intellectual disability. Additionally, they have well-documented physical traits, such as slanted eyes, a flat nasal bridge, and a single transverse palmar crease, which are characteristic facial features associated with the condition.

Cognitive impairment varies among individuals, but there is a general recognition that these individuals may require additional support in educational and social settings. Thus, identifying this association between an extra twenty-first chromosome and cognitive impairment, along with recognizable facial features, solidifies why this option represents the most accurate answer in the context of Down syndrome.