Entropy of a substance remains constant during a/an __________ change.

Answer: Option C

Since only for a reversible process we have relation :
$$TdS=\delta Q$$
Since for an adiabatic process $$\delta Q=0$$ $$\Rightarrow$$ for an reversible adiabatic process $$TdS=0.$$
So, entropy remains constant. Whereas for an irreversible adiabatic process $$TdS⩾0.$$
Hence entropy goes on increasing for an irreversible adiabatic process.