The Term Essential Functions of a Job,as Used in the Americans

The term essential functions of a job,as used in the Americans with Disabilities Act (ADA) ,best refers to:

A) tasks so designated by an employer as ancillary to the achievement of the job's long-term goals.
B) tasks so designated by the Equal Employment Opportunity Commission (EEOC) .
C) those tasks that are required to be performed in case of any emergencies.
D) those tasks that are fundamental, and not marginal or unnecessary, to fulfillment of the job's objectives.