Answer :

The fastest-growing age group in the United States is based on demographic studies and statistics. These studies take into account various factors such as birth rates, death rates, and longevity trends.

Recent demographic trends have shown an increase in the population of elderly individuals, mainly due to improved healthcare and living conditions which contribute to longer lifespans. As people live longer, the older age brackets grow more significantly.

In this context, considering the given options:
a. 80 and up
b. 20-35
c. 35-65
d. 0-15

We identify:
- The 80 and up group is likely to experience growth as people from younger age groups transition into this bracket over time, and with increasing life expectancy, more people reach and surpass the age of 80.

- The other age groups, while growing, do not grow as rapidly since they rely more heavily on birth rates and more immediate changes in the workforce.

Hence, the fastest-growing age group in the United States is:

a. 80 and up.