(a)(i) Habitat
The natural dwelling place of an organism. OR A place/locality where an organism lives successfully
(ii) Role of a decomposer in an ecosystem
Decomposers are organism which bring about the decay of dead parts/whole of plants/animals; to produce manure/humus; they release nutrients in them into the soil; which are absorbed by roots of plants; and are used to build up new plant tissues; they thus help in nutrient recycling; also release offensive gases/odour which pollute the air.
(b) Energy flow in a freshwater habitat;
Common aquatic plants/Pistia/Spirogyra/any correctly named aquatic plant; using energy from the sun; photosynthesize/produce food; the food is eaten by primary consumers/animals; small aqautic animals would be eaten by bigger aquatic animals/secondary consumers; tertiary consumer/a much bigger fish/any correctly named fish will feed on secondary consumer, in the process of feeding energy flows from one trophic level to another; decreases progressively; dead animals/plants decompose; energy is lost.
(c)(i) Harmful effects of microorganisms to plants

They cause diseases
They can cause undesirable qualities in plants/plant product
They cause spoilage
Reduction in yield

(ii) Beneficial effects of micro organisms to humans

Bacteria/decomposers help to decay/decompose dead organisms to release nutrients
Bacteria is used in cleaning/mopping up oil spillage
Used in the fermentation of foods/cocoa beans/garri/locust beans/kenkey production
Used to make vinegar
Used in making alcoholic beverages/wine/beer/spirit
Used in making some antibiotics/penicillin
Used in the manufacture of insulin
Used in making cheese/yoghurt/bread/Single Cell Proteins
Used in making flax/tanning of leather;