Which of the following is a free-living nitrogen-fixing bacterium?

Which of the following is considered to be the best chemical method of fixing atmospheric nitrogen?

Match the following and choose the correct combination from the options given.

Column $I$	Column $II$
$a.$ Potassium	$(i)$ Germination of pollen grains
$b.$ Molybdenum	$(ii)$ Involved in synthesis of auxin
$c.$ Boron	$(iii)$ Involved in stomatal movement
$d.$ Zinc	$(iv)$ Constituent of ferredoxin
$e.$ Sulphur	$(v)$ Component of nitrate reductase

Identify the correct pair:

Column-$I$	Column-$II$
$P$. Phosphorus	$I$. Splitting of water to liberate oxygen during photosynthesis
$Q$. Potassium	$II$. Constituent of cell membranes and all nucleotides
$R$. Magnesium	$III$. In more abundant quantities in the meristematic tissue,buds,leaves,and root tips
$S$. Manganese	$IV$. Constituent of the ring structure of chlorophyll and helps to maintain the ribosome structure

$P - Q - R - S$

Match the following columns:

Column-$I$	Column-$II$
$(A)$ Water splitting reaction	$(1)$ Boron
$(B)$ Uptake and utilisation of $Ca^{2+}$	$(2)$ Copper
$(C)$ Synthesis of auxin	$(3)$ Chlorine
$(D)$ Redox reaction	$(4)$ Zinc

Match the columns:
$I$ $II$
$(A)$ Magnesium $(i)$ Found in middle lamella
$(B)$ Sulphur $(ii)$ $A$ structural component of chlorophyll
$(C)$ Calcium $(iii)$ Required for enzyme activity
$(D)$ Iodine $(iv)$ Found in some amino acids
$(E)$ Manganese $(v)$ $A$ component of sugars
$(vi)$ Not important for plants