D. All of the above.
Gerontology is the study social, cultural, psychological, cognitive, and biological aspects of aging. Likewise, gerontologists are healthcare specialists who work with elderly patients. They provide services to people in nursing homes, senior citizen centers, and other similar facilities. So, we can assume that a gerontologist would be interested in the process of aging and would work with young-old, middle-old, and old-old people.
