Most oceanic gas hydrates form when
a. bacteria break down organic matter in seafloor sediments.
b. plant matter breaks down to form peat.
c. petroleum deposits seep into ocean floor sediment.
d. chemicals in ocean water precipitate out of solution.

a. bacteria break down organic matter in seafloor sediments.

Explanation:

The oceanic gas hydrates are formed from the methane gas. It develops when the organic matter generated after the decomposition of the plants and animals by bacteria or other microbes over the seafloor gets trapped in an ice lattice. The ice lattice is an reservoir of methane gas. These gas hydrates when break they leak out gases into the oceanic water.

Therefore on the basis of the above explanation, most oceanic gas hydrates form when the bacteria break down organic matter in seafloor sediments.