If the cells of root in wheat plant have $42$ chromosomes, then the number of chromosomes in the cell of pollen grain is

The correct option is A $21$
Chromosome number in pollen grains

Pollen is the male gametophyte having a haploid $\left(n\right)$ number of chromosomes, i.e., it has half the number of chromosomes compared to the diploid somatic cells.
Roots are diploid plant parts.
If the cells of root in a wheat plant have $42$ chromosomes, then the number of chromosomes in the nucleus of pollen grain is half of $42$, i.e., $21$.

Final answer
b. $21$