No need to apply any high concepts for this ques.

Just see it logiacally.

See in the figure, C has tetrahedral geometry and I hav shown it in the figure and the 4 lines are the bonds which a C atom can make.

 

Now U can clearly see that the bond which is most feasible is 1-A

Also the bonds which can be formed are 2-B, 3-D

But U can clearly see that 4-E is not possible.

 

If U want, try to change their orientations, but still U will find that U can form only atmost 3bonds and not the 4th one.

Hence C2 can't exist.

 

Hope U got it.