Option 1: Sodium is the correct answer. Sodium is a soft metal because it has a low hardness value. Softness is a measure of how easily a material can be scratched or deformed. Sodium has a relatively low hardness value on the Mohs scale, which measures the hardness of minerals and materials.

Option 2: Aluminium is not the softest among the given options. While aluminium is a relatively soft metal compared to others, such as steel, it is not as soft as sodium.

Option 3: Iron is not the softest among the given options. Iron is a harder metal than sodium and aluminium.

Option 4: Lithium is not the softest among the given options. While lithium is a relatively soft metal, it is harder than sodium. Lithium has a slightly higher hardness value on the Mohs scale than sodium.

In conclusion, sodium is the softest among the given options.