Q: Is methanol the only solvent that can be used instead of acetonitrile as the mobile phase in reversed-phase LC?

A:

It may be possible to use ethanol, isopropyl alcohol, acetone, or THF in addition to methanol.
With acetone, as the solvent itself has high UV absorbance, the baseline may not stabilize in UV detection. It can probably be used in refractive index detection (RID), but because it evaporates easily, there may be some influence on stability.
Care is required when using ethanol, isopropyl alcohol, or THF (tetrahydrofuran) because these solvents have a higher viscosity than acetonitrile and the pressure in the column may increase.