c
Bond length is defined as the distance between two atoms that are participating in a bond.

We can also understand the bond length with the help of the \(s\) character.

Higher the \(s\)-character lesser is the bond length.

This is because higher \(s\) character implies that the electrons that are bonded pairs are closely held together, thus making the bond length small as well as strong.

Hybridisation of \(C _2 H _6\) is \(sp ^3\)

The \(s\) character in \(C _2 H _6\) (Ethane) is: \(\frac{1}{4} \times 100=25 \,\%\) which is the least amongst the other given compounds.

Therefore the \(C - H\) bond length of \(C _2 H _6\) is the longest.

Hence it is the correct option.