Isentropic process means a constant __________ process.

Answer: Option C

Isentropic process is the process where entropy(s) remains constant. So, $$ds=0$$
Generally, $$ds$$ $$=\frac{\delta Q}{T}$$   for reversible process, so for reversible adiabatic process $$ds=0$$
Hence, reversible adiabatic process is called isentropic process.