D.

Pinus

Gametophyte is phase of life cycle in which the gametes, i.e. egg and sperm are produced. In bryophytes (Funaria and Marchantia) fertilisation is dependent on water for the transfer of sperm to the egg. The gametophyte are fetilized outside the body. Therefore, they have independent free-living gametophyte.
In this respect Pteridophyte are similar to bryophytes i.e they still have free-living gametophytes. Pinaceae (Pinus) has well developed gametophyte in which sperm cells are fertilised with egg cell within the ovule. Hence, pinus is the correct option.

D.

Mannitol is stored food in Rhodophyceae

Mannitol is stored food in Phaeophyceae. Red algae are the oldest groups of eukaryotic algae which includes marine algae especially seaweeds. The floridean starch is storage carbohydrate of red algae as grains outside the chloroplast.

C.

Methanogens

Methanogens are microorganisms taht produces methane as a by product of its metabolism. These are both archeans and obligate anaerobes. They are commonly found in wetlands, where they form marsh gas and in guts of animals such as ruminants and humans, where they are resposible for flatulence(burping)

B.

Indigofera

Keel is petal is vexillary aestivation, which is a characteristic of family -  Fabaceae, i.e. Indigofera tomato belongs to family - Solanaceae, tulip and while aloe to Liliaceae.
Papilionaceae (e.g. Indigofera, pea) is sub-family of Fabaceae, which has bilateral symmetry with corolla consist of 5 petals. It follows vexillary aestivation. A single upper posterior petal is known as banner.  Two lateral compressed petals called wings and a pair of anterior small fused petal forms keel. Keel has concave sides which make them fused at bases. 

Hence, keel is the characteristic feature of flower of family - Papilionaceae. e.g. Indigofera.

B.

grana

The structures that are formed by stacking of organised flattened membranous sacs (thylakoids) in the chloroplasts are grana. Chloroplasts are organelles found in plant cells that conduct photosynthesis. It is made up of three layers (membranes).
(i) Smooth outer membrane
(ii) Smooth inner (middle) membrane
(iii) Innermost membrane
It has thylakoid system, which is surrounded by the stroma and grana.
Stroma is a alkaline, aqueous fluid which is protein rich.
Grana is formed by stacking of organised flattened membranous sacs called thylakoid.