Explanation

Acetylene has a triple bond between two adjacent carbon atom 

And the bond length is even shorter. 

it is closer to 120 pm

Each carbon atom in a diamond is surrounded by 4 other carbon atoms.

And connected to them by a strong single covalent bond- the strongest type of chemical bond.

The bond length is 154 pm.

Hence the shortest C - Bond distance is found in acetylene which is 120 pm.

Additional information:

Note that as the C-C bond length is shortening, the bond length of C-H is alose decreasing.

This is due to the presence of hyperconjugation in alkene and alyknes 

Where each bond attains a partial double bond character.