Answer: Option A. -> True
:
A
Nitrogen is the most abundant element in the atmosphere and it forms a major component of several important molecules.
However, the availability of nitrogen in the soil is limited because plants—the primary food producers—lack the ability to fix atmospheric nitrogen. They depend upon nitrogen-fixing soil bacteria to 'fix' (think modify) the nitrogen for them to use unlike carbon or hydrogen which they can modify themselves.
This makes nitrogen a limiting nutrient even though it's the most abundant in the atmosphere. Ironic, isn't it?

Answer: Option D. -> Plants accumulate nitrogen in the form of ammonium
:
D
Most of the plants assimilate nitrate as well as ammonium ions. Ammonium is toxic and hence it can not be accumulated in plants.

Answer: Option C. -> the amino acids, proteins, enzymes, nucleic acids hormones and pigments
:
C
The nitrates and ammonium ions absorbed by plants is converted into amino acids, proteins, enzymes, nucleic acids, pigments, hormones etc.,

Answer: Option A. -> Both A & R are true and R explains A
:
A
Nitrogenase can only break N ≅ N bond and it is absent in higher plants, hence they cannot utilize $N_2$ present in the air.

Answer: Option B. -> Glutamic acid
:
B
From glutamic acid, more than 17 amino acids are formed through transamination which isthe transfer of an amino group from one molecule to another, especially from an amino acid to a keto acid.

Answer: Option B. -> Alcohol dehydrogenase
:
B
Alcohol dehydrogenase (ADH) have a structural zinc site. This $Z{n}^{2+}$ ion plays a structural role and is crucial for protein stability of the ADH enzyme.

Answer: Option D. -> Fe
:
D
Ironis involved when aplantproduces chlorophyll, which gives theplantoxygen as well as its healthy green color. This is whyplantswith anirondeficiency, or chlorosis, show a sickly yellow color to their leaves.

Answer: Option D. -> Deficiency of a non-mobile mineral nutrient
:
D
A deficiency in a non-mobile mineral nutrient causes yellowing of theplant’s younger leaves, not the older ones.

Answer: Option A. -> A - 3, B - 1, C - 4, D - 2
:
A
Nitrogen is constituent of proteins, enzymes, hormones, vitamins etc.Phosphorus is a component of cell membrane, nucleic acids etc. Potassium maintains ion balance and helps stomatal opening and closing. Calcium plays important role in spindle formation and cell plate formation.

:
Rhizobium is present in the root nodules of legumes like sweet pea, garden pea, broad bean, etc.,where the interactive association between these two species is symbiosis that benefits both the species for survival.