Question

Nodule formation of legume roots is reduced during deficiency of
(a) Sulphur
(b) Boron
(c) Nitrogen
(d) Both A and B

Answer 1

The deficiency is caused by elements in which one is non-metallic, yellow crystalline solid at room temperature while the other one is a metalloid which is a dark colored powder and does not react with oxygen.

Complete answer:
Boron and Sulphur are both micronutrients needed for the plant in minimal quantities for proper growth and development. Under conditions with minimal nitrogen availability in the soil, leguminous plants are capable of symbiotic relation with host specific rhizobium bacteria which can fix nitrogen from the atmosphere in the soil. This close association of bacterial strains from the root nodules of the plants.
In the root nodules of legumes, the atmospheric ${N_2}$ is converted into Ammonia ${NH_3}$ which is soluble and can be easily taken up by the plants. This Ammonia is used in the formation of Amino acids that are constituents of proteins, DNA and RNA molecules, as well as ATP which is the major energy subunit.
Cobalt is generally essential in all legumes for proper uptake of sulphur and also required for the formation of Leghemoglobin which is an important component of energy transfer and responsible for growth of nodules in leguminous plants, specifically lentils.
In Nodulated legumes sulfur deficiency causes similar effects along with limitation of nitrogen fixation and altered nodule metabolism.

So, the correct answer is 'Both A and B'.

Note:
Though majority of plants with ability to to form nitrogen-fixing root nodules are from the leguminous family Fabaceae, there exist some exceptions to this:
-Parasponia, a genus in the Cannabaceae are also able to form nitrogen-fixing nodules in symbiosis with rhizobia.
-Actinorhizal plants like the Alder and Bayberry are also able to form nitrogen-fixing nodules, with a symbiotic association with bacteria Frankia. These are 25 genera of plants that are distributed to about 8 plant families.