Answer :

Since chlorine and argon are propellant and sulfur is solid. Then the answer is 2) Bromine. 

Answer: Option (2) is the correct answer.

Explanation:

STP means standard temperature and pressure.

At STP chlorine exists as a gas, sulfur exists as a solid and argon also exists as a gas.

Whereas bromine exists as a liquid at STP.

Thus, we can conclude that out of the given options bromine is the element which exists as a liquid at STP.