According to valence bond theory

(A) A sigma bond is stronger than a pi-bond

(B) Sigma bonds are always the first bond to form so every single covalent bond is a sigma bond.

(C) Overlapping of atomic orbitals is to a larger extent in a sigma bond than a pi-bond. 

In the case of the sigma bond, the overlapping of orbitals takes place to a larger extent. Hence, the sigma bond is stronger in comparison to the pi bond because, in π bond, the extent of overlapping occurs to a smaller extent.

Sigma bonds are always the first bond to form, so every single covalent bond is a sigma bond. Pi bonds are the second and third bonds to form, so double and triple bonds have the initial sigma bond and then one and two pi bonds, respectively.