The non-metal which is a liquid at room temperature:
A. Chlorine
B. Nitrogen
C. Bromine
D. Hydrogen

Hint: Bromine is a member of the halogen group. It is heavy, volatile, mobile and dangerous reddish brown liquid.

Complete answer:
Bromine is the only non-metal that is in liquid at room temperature. It is one of the only two elements on the periodic table that are liquids at room temperature other than Mercury.
Corresponding to periodic trends it is intermediate in electronegativity between chloride and iodine. It is less reactive than chlorine and more reactive than iodine. Conversely, the bromine ion is a weaker reducing agent than iodine , but stronger than chloride. These similarities led to chlorine, bromine and iodine together being classified as one of the original triads of Johann Wolfgang Dobereiner whose work foreshadowed the periodic law for chemical elements.

Now, let's find out the actual reason behind bromine being a liquid.
Bromine is a liquid because the intermolecular forces are strong enough so that it does not evaporate. Bromine forms diatomic molecules and Van der Waals interactions are sufficiently strong, so it exists as a liquid.
Whereas nitrogen, hydrogen and chlorine are gases at room temperature.

Therefore, option C is the correct answer.

Note: Do not get confused with mercury, as Mercury is also liquid but it's a metal. The other metals are solids at room temperature, including carbon and sulphur.