If a disability keeps a person from performing the __ of a job, the person does not have to be hired or retained.

The correct answer would be, Essential Functions.

Explanation:

If a disability keeps a person from performing the essential functions of a job, the person does not have to be hired or retained.

Disability should not come between the performing of tasks or doing a job. For example if a person is disabled from legs and he cannot move around, and his job is also a desk job which does not require to move around frequently, then there shouldn't be anything which keeps him in doing his essential job duties or functions. One should not take his disability between his work. But if a disability keeps a person from performing the essential functions of a job, the person should not be hired or retained. For example, if there is a job which require frequent move around, then a physically disabled person is not fit for this job.

b. take periodic breaks and alternate between tasks.  

will increase work efficiency, and avoid health issues.