ANS:C - both (a) and (b).

Let's break down both scenarios: (a) Above which it will always become damp: This statement refers to the critical humidity level of a solid salt. When the humidity in the surrounding environment rises above the critical humidity level of the salt, the salt will absorb moisture from the air until it reaches equilibrium with the surrounding humidity. As a result, the salt will become damp or dissolve, depending on its solubility in water. This is because the salt has a hygroscopic nature, meaning it has a strong affinity for water molecules and readily absorbs moisture from the air when exposed to high humidity conditions. (b) Below which it will always stay dry: Conversely, when the humidity in the surrounding environment is below the critical humidity level of the salt, the salt will generally remain dry. In this scenario, the surrounding air does not contain enough moisture for the salt to absorb, so it remains in a dry state. This is particularly true for salts with low hygroscopicity or solubility in water, as they are less likely to absorb moisture from the air and become damp. So, both statements (a) and (b) describe the behavior of a solid salt with respect to humidity levels:

• (a) describes the behavior when humidity levels rise above the critical humidity, leading to the salt becoming damp.
• (b) describes the behavior when humidity levels remain below the critical humidity, resulting in the salt staying dry.