1, 2 and 3 are correct.
Biogas is generated during anaerobic digestion when microorganisms break down organic materials in the absence of air (or oxygen). Biogas is mostly methane (CH4) and carbon dioxide (CO2), with very small amounts of water vapor and other gases. The carbon dioxide and other gases can be removed, leaving only the methane. Methane is the primary component of natural gas.

Only 2 is correct.
Epiphytic (plants growing on other plants including those growing on rock boulders and often termed lithophyte), Terrestrial (plants growing on land and climbers)
Mycoheterotrophic (plants which derive nutrients from mycorrhizal fungi that are attached to the roots of a vascular plant).

Ocean is a self-sustaining ecosystem and covers nearly 2/3rd of earth’s area. It is the oldest ecosystem. It is least affected by natural calamities.
It is a highly diverse (from phytoplankton to sea grass to whales) and productive ecosystem and contains earliest evolutionary life forms. The evolutionary history has also been more stable than in terrestrial environments.

1, 2 and 3 are correct.
What is so special about tropics that might account for their greater biological diversity? Ecologists and evolutionary biologists have proposed various hypotheses; some important ones are
(a) Speciation is generally a function of time, unlike temperate regions subjected to frequent glaciations in the past, tropical latitudes have remained relatively undisturbed for millions of years and thus, had a long evolutionary time for species diversification,
(b) Tropical environments, unlike temperate ones, are less seasonal, relatively more constant and predictable. Such constant environments promote niche specialisation and lead to a greater species diversity and
(c) There is more solar energy available in the tropics, which contributes to higher productivity; this in turn might contribute indirectly to greater diversity.

1, 2 and 3 are correct.
Some examples of recent extinctions include the dodo (Mauritius), quagga (Africa), thylacine (Australia), Steller’s Sea Cow (Russia) and three subspecies (Bali, Javan, Caspian) of tiger.
The sloth bear is a myrmecophagous bear species native to the Indian subcontinent. It is listed as Vulnerable on the IUCN Red List, mainly because of habitat loss and degradation.