The carbon-carbon bond length is inversely related to the bond order. Higher the bond order, shorter the bond length.

Let's analyze the carbon-carbon bonds in each option:

• A) ${\mathrm{C}}_6{\mathrm{H}}_6$ (Benzene): Benzene has delocalized $\pi$ electrons, resulting in carbon-carbon bonds with a bond order of approximately 1.5 (intermediate between a single and a double bond). The C-C bond length is about 1.39 Å.
• B) ${\mathrm{C}}_2{\mathrm{H}}_6$ (Ethane): Ethane has a carbon-carbon single bond (C-C). The bond order is 1. The C-C bond length is about 1.54 Å.
• C) ${\mathrm{C}}_2{\mathrm{H}}_4$ (Ethene): Ethene has a carbon-carbon double bond (C=C). The bond order is 2. The C=C bond length is about 1.34 Å.
• D) ${\mathrm{C}}_2{\mathrm{H}}_2$ (Ethyne): Ethyne has a carbon-carbon triple bond (C$\equiv$C). The bond order is 3. The C$\equiv$C bond length is about 1.20 Å.

Comparing the bond lengths, the carbon-carbon triple bond in ethyne (1.20 Å) is the shortest.

Correct Answer: (D)