In FEM, which option is a negative aspect of the mixed formulation?

(a) Presence of zeros on matrix diagonals corresponding to pressure variable

(b) Involves a natural and direct formulation

(c) It is a velocity-pressure formulation

(d) It is similar to the Lagrange multiplier method

The question was posed to me in semester exam.

This is a very interesting question from Penalty topic in section Flows of Viscous Incompressible Fluids of Finite Element Method