Which substance (P, Q, R or S, by melting and boiling point) is a liquid at room temperature and evaporates rapidly?

Room temperature is about 25 °C. A substance is a liquid when 25 °C lies between its melting point and its boiling point.

Q melts at -8 °C and boils at 40 °C. So at 25 °C it is above its melting point and below its boiling point, which makes it a liquid. Its boiling point is also low, so it is close to boiling and evaporates quickly in open air.

P boils at -25 °C, so it is already a gas at room temperature. S melts at 44 °C, so it is a solid. R is a liquid too, but it boils at 150 °C, so it evaporates slowly.