Explanation:

Introduction:
The distance between two adjacent carbon atoms in a molecule depends on the type of bond between them. Different types of bonds have different bond lengths. The bond length is the distance between the centers of the two atoms connected by the bond.

Bond Length in Organic Molecules:
In organic molecules, carbon atoms can be connected by single, double, or triple bonds. The bond length increases with the number of bonds between the carbon atoms. Single bonds are longer than double bonds, and double bonds are longer than triple bonds.

Comparing Molecules:
Let's compare the four molecules mentioned in the options to determine the molecule in which the distance between two adjacent carbon atoms is largest.

1. Benzene:
Benzene is a cyclic aromatic compound with a hexagonal ring of carbon atoms. Each carbon atom in benzene is bonded to three other carbon atoms, resulting in alternating single and double bonds. The bond length between adjacent carbon atoms in benzene is intermediate between a single bond and a double bond.

2. Ethyne:
Ethyne, also known as acetylene, is an unsaturated hydrocarbon with a triple bond between two carbon atoms. The bond length between adjacent carbon atoms in ethyne is shorter than in benzene.

3. Ethene:
Ethene, also known as ethylene, is an unsaturated hydrocarbon with a double bond between two carbon atoms. The bond length between adjacent carbon atoms in ethene is shorter than in benzene but longer than in ethyne.

4. Ethane:
Ethane is a saturated hydrocarbon with all single bonds between carbon atoms. The bond length between adjacent carbon atoms in ethane is the longest among the four molecules mentioned.

Conclusion:
Therefore, the molecule in which the distance between two adjacent carbon atoms is largest is ethane (option D).