For an adiabatic process involving an ideal gas, which quantity is constant?

Adiabatic process means no heat exchange ($dQ = 0$). The first law plus the ideal-gas equation give $PV^\gamma = \text{constant}$, where $\gamma = C_p / C_v$ is the adiabatic index (about $1.4$ for diatomic gases, $5/3$ for monoatomic). Option A ($PV = $ constant) is the isothermal law, not the adiabatic one. The exponent $\gamma > 1$ is what makes adiabatic curves steeper than isothermal curves on a $P$-$V$ diagram.