In FEM, what is the characteristic of the problem with the functional Iv(v) representing governing equations of steady viscous incompressible flows?

(a) It is subjected to constraint \(\frac{\partial v_x}{\partial x}+\frac{\partial v_y}{\partial y}\)=0

(b) It is an unconstrained problem

(c) It can be reformulated as a constrained one, by using the penalty method

(d) It cannot be reformulated as an unconstrained one, by using the penalty method

The question was posed to me during an interview for a job.

This intriguing question comes from Penalty topic in portion Flows of Viscous Incompressible Fluids of Finite Element Method