State the conditions under which nitrogen gas can be liquefied.

Gases can be liquefied by applying high pressure. As gases are highly compressible, on applying high pressure, the molecules of a gas would come close, inter molecular forces would increase and it will condense into a liquid. By lowering the temperature:Gases can be converted into liquids by simple cooling.

the old alchemists made significant contributions to science even though there starting premises were off a bit. their conclusions after failure would steer them into better directions for investigation. science is no more than close observation and the initial mistakes result in rethinking and retrying experiments.

the answer is definitely c.