'Aerial metagenomics' best refers to which one of the following…

Collecting DNA samples from air in a habitat at one go
Understanding the genetic makeup of avian species of a habitat
Using air-borne devices to collect blood samples from moving animals
Sending drones to inaccessible areas to collect plant and animal samples from land surfaces and water bodies

Answer

Collecting DNA samples from air in a habitat at one go

(a) Collecting DNA samples from air in a habitat at one go — Correct. Aerial metagenomics means sampling environmental DNA (eDNA) from air and analyzing the mixed genetic material of organisms present in that habitat.
(b) Understanding the genetic makeup of avian species of a habitat — Incorrect. This is limited to birds, whereas metagenomics studies mixed DNA from many organisms, not just avian species.
(c) Using air-borne devices to collect blood samples from moving animals — Incorrect. Blood sampling from animals is not metagenomics; metagenomics deals with community DNA from environmental samples.
(d) Sending drones to inaccessible areas to collect plant and animal samples from land surfaces and water bodies — Incorrect. Drone-based collection may be a field method, but aerial metagenomics specifically refers to DNA collected from air itself.

Therefore, the correct answer is (a).