Which bond is strongest and why?

a. C=C

C=N

C=O

C=S

For this you should look at each elements' electronegativity. The closer the difference is to zero is the most covalent and strongest. Even without looking at their individual electronegativities, I know that the Carbon-Carbon bond will be the strongest. They have the same electronegativity so when you subtract them you get 0. So, your answer is A, the double bonded C2