One characteristic that distinguishes a population in an extinction vortex from most other populations is that:
a) its habitat is fragmented.
b) it is a rare, top-level predator.
c) its effective population size is much lower than its total population size.
d) its genetic diversity is very low.

D - Its genetic diversity is very low

Explanation:

An extinction vortex can be defined as the process which populations in decline pass through to drive them into a vortex of smaller populations, endangering their survival as they tread the path to extinction.

Populations that fall to extinction vortex already have small populations which is usually as a result of certain powerful genetic factors, which cause the populations to continue to decline in size.

During genetic drift which occurs in all populations, small populations tend to lose genetic diversity as a result of the random nature of gamete sampling.

This is because any change in alleles in a small populations can have dastardly effects on the populations size due to a small gene pool.