Answer: Option A. -> humans
:
A
Chemicals present in fertilisers and pesticides can enter inside soil or water bodies. From there, they can be taken up by aquatic or terrestrial plants and animals. Since, these chemicals are not degradable. They get accumulated at each trophic level. Human beings, are the top carnivores hence occupy the top level in the food chain. So, the maximum concentration of these chemicals will be accumulated in their bodies.

Answer: Option B. -> Product with carbon as major constituent
:
B

Compost is made out of organic substances which have carbon as a major constituent. In other words, biodegradable substances are required for composting. Thus, the product should be made of biodegradable substances, usually organic.

Answer: Option B. -> Both A and R are true
:
B

The organic matter is decomposed by the microorganisms present in the soil or water. They do so with the help of enzymes that help them to digest complex organic molecules into simpler ones. But the micro-organisms present on the earth today lack such enzymes which will help them to digest plastic. Hence, plastic cannot be decomposed and becomes non-biodegradable. 

Answer: Option B. -> Water
:
A, B, and D
Abiotic or non-living things are the physical components of the environment. A few examples of abiotic factors are water, soil, air, heat, and light.  
On the other hand, biotic factors are the living components of an ecosystem such as plants, animals and micro-organisms.
Here, bird is a biotic component.

Answer: Option B. -> By adding more plants and animals
:
B

An aquarium can be made self-sustained by adding more plants and animals in it which would help in recycling of certain substance like carbon dioxide, wastes etc which would otherwise pollute the environment of the aquarium.

Answer: Option A. -> True
:
A
A food chain is a series of organisms where each organism becomes a food source for the next one. For example, grass becomes food for grasshopper and grasshopper for mouse and mouse for owl. In other words, a food chain is a basic network that shows the linear flow of nutrients and energy from one trophic level to another.

Answer: Option A. -> Jaguars
:
A, B, C, and D
Organisms that obtain energy (i.e.,nutrition) from organic compounds or materials are called heterotrophs.
In other words, they are unable to produce their own food and therefore, have to consume or ingest organic compounds as a source of energy.
Also, heterotrophs occupy the upper levels of the food web and that their survival is dependent on the producers (autotrophs). 
Jaguars and sharks come under the category of carnivores and are predators who hunt to feed on their prey.
Most humans and bears are omnivores, who eat both plants and animals as well.

Answer: Option C. -> 430 ppmv
:
C

430 ppmv is the current concentration of $C{O}_2$  in atmosphere. The global concentration of carbon dioxide in the atmosphere – the primary driver of recent climate change – has reached over 400 parts per million (ppm) for the first time this year. The safe level of carbon dioxide in the atmosphere is 350 parts per million.

Answer: Option B. -> 2
:
B

Paper is biodegrdable and hence the bags made out of paper are also biodegradable. Polyethene bags and waste parts of mobile phones or any other electronic devices (e-waste) are non-biodegradable.

Answer: Option B. -> they act as decomposers.
:
B

Microorganisms like bacteria and fungi, act as decomposers as they break down complex organic substances into simpler inorganic substances that go into the soil. These inorganic substances are then recycled by plants.
Decomposers are considered to be of great importance as in their absence, earth would be filled with dead bodies.