The change in __________ is equal to the reversible work for compression in steady state flow process under isothermal condition.

Answer: Option C

Gibbs free energy:
$$\begin{array}{rl}& G=H-TS\\ & \Rightarrow -dG=W_R-PdV\end{array}$$
So, the decrease in Gibbs energy represents maximum useful work (excluding the expansion work) in a reversible process under isothermal and isobaric conditions.