Answer:

Explanation:
In order to determine which compound will have the least hindered rotation about the carbon-carbon bond, we need to analyze the structure and determine the factors that contribute to hindered rotation.

Ethane:
Ethane has a simple structure consisting of two carbon atoms connected by a single bond. The rotation about the carbon-carbon bond is relatively free because there are no bulky substituents or functional groups present that would hinder the rotation. Therefore, ethane will have the least hindered rotation among the given options.

Ethylene:
Ethylene has a double bond between the two carbon atoms. The double bond restricts the rotation about the carbon-carbon bond because it involves the breaking and reforming of the pi bond. This bond rotation requires a significant amount of energy and is therefore hindered compared to the rotation in ethane.

Acetylene:
Acetylene has a triple bond between the two carbon atoms. Similar to ethylene, the presence of multiple bonds restricts the rotation about the carbon-carbon bond. The rotation would require the breaking and reforming of multiple bonds, which is energetically unfavorable and hindered.

Hexachloroethane:
Hexachloroethane has six chlorine atoms attached to the carbon atoms. The bulky chlorine atoms create steric hindrance, which hinders the rotation about the carbon-carbon bond. The presence of the bulky chlorine atoms restricts the movement around the bond and makes the rotation energetically unfavorable.

Therefore, among the given options, ethane will have the least hindered rotation about the carbon-carbon bond.