Sodium borate is a basic substance in a 0.1 M solution.

Explanation:
- Sodium borate, also known as borax, is an inorganic compound that consists of sodium ions and borate ions.
- In solution, the borate ion (B(OH)4-) can accept a proton (H+) from water, making it a weak base.
- The equilibrium reaction is as follows:
B(OH)4- + H2O ⇌ B(OH)3 + OH-
- The hydroxide ion (OH-) produced in this reaction is responsible for the basic nature of the solution.
- In a 0.1 M solution of sodium borate, there is a sufficient concentration of borate ions to produce a basic solution.

Comparison with other substances:

a) Sodium borate (option A) - Basic
- As explained above, sodium borate is a weak base and can produce hydroxide ions in solution.

b) Ammonium chloride (option B) - Acidic
- Ammonium chloride is an inorganic compound that dissociates into ammonium ions (NH4+) and chloride ions (Cl-) in water.
- The ammonium ion can act as a weak acid, donating a proton to water and producing hydronium ions (H3O+).
- The equilibrium reaction is as follows:
NH4+ + H2O ⇌ NH3 + H3O+
- The hydronium ions make the solution acidic.

c) Calcium nitrate (option C) - Neutral
- Calcium nitrate is an inorganic compound that dissociates into calcium ions (Ca2+) and nitrate ions (NO3-) in water.
- Neither the calcium ion nor the nitrate ion has the ability to act as an acid or a base in this solution.
- Therefore, the solution remains neutral.

d) Sodium sulphate (option D) - Neutral
- Sodium sulphate is an inorganic compound that dissociates into sodium ions (Na+) and sulphate ions (SO4 2-) in water.
- Neither the sodium ion nor the sulphate ion has the ability to act as an acid or a base in this solution.
- Therefore, the solution remains neutral.

In conclusion, the 0.1 M solution of sodium borate is basic because the borate ion can accept a proton from water, producing hydroxide ions. The other substances mentioned (ammonium chloride, calcium nitrate, and sodium sulphate) do not have the ability to produce hydroxide ions and therefore do not make the solution basic.