In the reaction ${\text{CH}}_3\text{C}\equiv \text{CH}\stackrel{\text{Red hot iron tube, }873\text{K}}{\to }A$. The number of ($\sigma$) bonds present in the product (A) is:

Propyne (${\text{CH}}_3\text{C}\equiv \text{CH}$) undergoes cyclic trimerization when passed through a red-hot iron tube.The product (A) is Mesitylene (1,3,5-Trimethylbenzene).Structure of Mesitylene (${\text{C}}_9{\text{H}}_{12}$):Benzene ring: 6 $\text{C-C}$ $\sigma$ bonds.Three Methyl groups attached to ring: 3 $\text{C-C}$ $\sigma$ bonds.Hydrogens on the ring (at positions 2, 4, 6): 3 $\text{C-H}$ $\sigma$ bonds.Hydrogens in three methyl groups: $3\times 3=9$ $\text{C-H}$ $\sigma$ bonds.Total $\sigma$ bonds $=6+3+3+9=21$.