A metric that measures the decline in population sizes, shrinkage of species, and geographic range is .

Living Planet Index

Explanation:

The Living Planet Index is a survey measure of global biodiversity status based on the population abundance of vertebrate species in terrestrial, freshwater and marine habitats. We have to consider that a population is defined as a species within a geographical area. It is used as a source of information to identify threats at the population level for each species on Planet Earth.  

With this analysis, an overall decline of about 60 percent in population size has been seen between 1970 and 2014. The decline in species populations is particularly greatest in the tropics, with South America and Central America suffering the most dramatic decline. The numbers of freshwater species have also declined dramatically. However, measuring biodiversity (all the varieties of life that can be found on Earth and the relationships between them) is complex, so the index should consider indicators to measure changes in species distribution, extinction risk and changes in community composition.  

The goal of the Living Planet Index is to achieve and promote global conservation of animals and their habitats.

the answer is b.) hydrogen bonds between bases.