Interpret the question

The question asks: when a system containing solution 1 becomes solution 2, which property changes?

Since the two states are different solution states, we look for a property that is necessarily a thermodynamic state function whose value depends on the state of the system.

Check each option

Option A: Molar heat capacity

Molar heat capacity depends on the nature of the system and conditions, but it is not the standard quantity used to indicate spontaneous change between two solution states. Also, it may or may not change depending on the compositions and conditions.

Option B: Concentration

Concentration can change if composition changes, but the wording of the question is asking about the thermodynamic property associated with the change of one solution state into another. Concentration is a compositional descriptor, not the key criterion for the driving force of such a transformation.

Option C: Gibbs free energy

Gibbs free energy $G$ is a state function. If the system changes from solution 1 to solution 2, then in general

so the Gibbs free energy changes.

This is the most definite and thermodynamically correct choice.

Option D: Density

Density may or may not change depending on the nature of the two solutions. It is not guaranteed solely from the statement unless more information is given.

Conclusion

The property that will change when the system changes from solution 1 to solution 2 is:

Compare with stored correct answer

Stored correct answer: C

Derived answer: C

They match.