Which of the following is (are) the property(ies) of ionic compounds?

Both (a) and (b) 

The correct answer is option 3. Both (a) and (b).

Both options (a) and (b) are properties of ionic compounds:

(a) Ionic compounds typically have high melting and boiling points. This is because ionic compounds are held together by strong electrostatic forces of attraction between positively and negatively charged ions, known as ionic bonds. These bonds require a significant amount of energy to break, resulting in high melting and boiling points.

(b) Ionic compounds can conduct electricity in solution or in the molten state. In the solid state, ions are held in a fixed position and cannot move, so ionic compounds do not conduct electricity. However, when dissolved in water or melted, the ions become mobile and are able to carry electric current. This is because the movement of ions allows for the flow of electric charge.

Therefore, the correct answer is Both (a) and (b).