Q.In an adiabatic process, which of the following is true?

Any operation that changes a system's state is a thermodynamic process. The most fundamental split is reversible (system and surroundings stay in mutual equilibrium at every instant, so the process could in principle be run infinitesimally slowly and reversed with zero net change anywhere in the universe) versus irreversible (system and surroundings are NOT in equilibrium during the change, and the process cannot be fully undone -- every real, spontaneous process in nature is irreversible; "reversible" is an idealised limiting case used to derive exact formulas).

Processes are further named by which variable is held fixed: adiabatic ($q=0$, thermally insulated -- if the system does work adiabatically its own temperature falls, since it cannot replace the lost energy with incoming heat); isothermal ($dT=0$, run inside a thermostat so heat DOES flow to keep temperature constant); isobaric ($dP=0$); isochoric ($dV=0$, e.g. combustion inside a sealed bomb calorimeter); and cyclic (the system returns to its exact starting state, so every state function -- $U$, $H$, $P$, $V$, $T$ -- returns to its original value over the complete cycle, even though none of them need be zero at intermediate points).