In a reversible adiabatic change $\Delta S$ is

$(a)\;\text{infinity} \\(b)\;\text {zero} \\(c)\;\text{equal to C_vdT} \\(d)\;\text{equal to nRln} \frac{V_2}{V_1}$

In a reversible adiabatic change $\Delta S$ is zero
Hence b is the correct answer.