Binding of complement to bacterial cell surface is called:
The microorganisms are coated with specific proteins, opsonins from the serum and the process is called opsonisation (meaning preparing for eating). 
Opsonins establish a bond between bacteria and the cell membrane of phagocytic cell. 
The main opsonins present in the serum and their corresponding receptors on the surface of phagocytic cells (PMNs or macrophages) are as under:

i) IgG opsonin is the Fc fragment of immunoglobulin G; it is the naturally-occurring antibody in the serum that coats the bacteria, while the PMNs possess receptors for the same.
ii) C3b opsonin is the fragment generated by activation of complement pathway. It is strongly chemotactic for attracting PMNs to bacteria.
iii) Lectins are carbohydrate-binding proteins in the plasma which bind to bacterial cell wall.
Opsonisation
Complement activation
Complement stabilization
None of the above