IUPAC says it’s propane, though if one wants to be cute, dimethyl methane will do. n-propane is unnecessary, since there’s only one isomer.
The physical state of propane depends on the pressure-- at normal atmospheric pressure, propane boils at -42 Celsius.
LPG is a mixture of butane (4 Carbons) and propane (3 Carbons) stored under enough pressure to partially liquify.
