The atoms in a single bond can rotate about the internu- clear axis without breaking the bond. the atoms in a double bond and a triple bond cannot rotate about the internuclear axis unless the bond is broken. why?

Double or triple bonds contains one or two pi bonds respectively along with one sigma bond. Pi bonds are formed by the sideways overlapping and thus they are non-symmetrical about the internuclear axis and thus it breaks if the bond is being rotated.

Explanation:

The atoms which comprises of single bond is formed by sigma bond. Sigma bond is formed by the internuclear overlapping of the atomic  orbitals. They are formed due to the head on overlapping of the atomic orbitals. Since, the bond is symmetrical about the internuclear axis, it can rotate without the breakage of the bond.

On the other hand, double or triple bonds contains one or two pi bonds respectively along with one sigma bond. Pi bonds are formed by the sideways overlapping and thus they are non-symmetrical about the internuclear axis and thus it breaks if the bond is being rotated.

i think it's letter a.