Alkyl halides are colourless when pure. However, bromides and iodides develop colour when exposed to light. Many volatile halogen compounds have sweet smell.

Melting and boiling points
Methyl chloride, methyl bromide, ethyl chloride and some chlorofluoromethanes are gases at room temperature. Higher members are liquids or solids. As we have already learnt, molecules of organic halogen compounds are generally polar. Due to greater polarity as well as higher molecular mass as compared to the parent hydrocarbon, the intermolecular forces of attraction (dipole-dipole and van der Waals) are stronger in the halogen derivatives. That is why the boiling points of chlorides, bromides and iodides are considerably higher than those of the hydrocarbons of comparable molecular mass. The attractions get stronger as the molecules get bigger in size and
have more electrons.

Density
Bromo, iodo and polychloro derivatives of hydrocarbons are heavier than water. The density increases with increase in number of carbon atoms, halogen atoms and atomic mass of the halogen atoms.

Solubility
The haloalkanes are very slightly soluble in water. In order to dissolve haloalkane in water, energy is required to overcome the attractions between the haloalkane molecules and break the hydrogen bonds between original hydrogen bonds in water. As a result, the solubility of haloalkanes in water is low. However, haloalkanes tend to dissolve in organic solvents because the new intermolecular attractions between haloalkanes and solvent molecules have much the same strength as the ones being broken in the separate haloalkane and solvent molecules.

Which of the following haloalkanes exist as liquids at room temperature? 

Propyl bromide

The correct answer is option 4. Propyl bromide.

Out of the listed haloalkanes, only propyl bromide exists as a liquid at room temperature.

Here is the breakdown:

Methyl chloride \((CH_3Cl)\): This has a boiling point of -23.7°C and a melting point of -97.7°C. Since room temperature is typically around 20-25°C, methyl chloride would be a gas at that temperature.

Methyl bromide \((CH_3Br)\): Similar to methyl chloride, methyl bromide has a boiling point of 3.5°C and a melting point of -93.7°C. It too remains a gas at room temperature.

Propyl chloride \((C_3H_7Cl)\): Propyl chloride has a boiling point of 46.6°C and a melting point of -122°C. While its melting point is well below room temperature, its boiling point is still slightly higher than typical room temperature. There's a chance it could exist as a liquid depending on the specific room temperature. However, it's borderline.

Propyl bromide \((C_3H_7Br)\): With a boiling point of 71.5°C and a melting point of -111.2°C, propyl bromide has a boiling point well above room temperature. This ensures it remains a liquid under those conditions.

Therefore, the most reliable option for a liquid at room temperature is propyl bromide. Propyl chloride might be a liquid depending on the exact room temperature, but it's less certain.