One flask immediately emits a gas once the stopper is removed what does the flask contain a. acetone b. diethyl ether c. butane d. chloroform

The answer is; C

This is due to the fact that the boiling point of butane is -5 degrees centigrade. These elements evaporate at room temperature (room temperature is usually around 20 degrees centigrade) which is past the boiling point of butane. A substance turns to gas when it passes it boiling point temperatures hence butane seems to sublimate when its flask is opened.