According to scientific definition, which of the following is a pure substance?

Common salt

The correct answer is option 2. Common salt.

When a scientist says that something is pure, it means that all the constituent particles of that substance are the same in their chemical nature. A pure substance consists of a single type of particle.

Common salt (NaCl) i.e., sodium chloride is itself a pure substance and cannot be separated by physical process into its chemical constituents.