The standard solution of sodium hydroxide cannot be prepared by direct weighing of cold sodium hydroxide because it is:
(A) a solid
(B) a liquid
(C) deliquescent
(D) none of the above

Hint: NaOH is a highly hygroscopic substance. A hygroscopic substance is able to absorb or adsorb water from its surroundings. Typically, this occurs at or near ordinary room temperature. Most hygroscopic materials are salts.

Complete step by step answer:
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations \[N{{a}^{+}}\]
 and hydroxide anions \[O{{H}^{-}}\] .
Sodium hydroxide is a highly caustic base and alkali that decomposes proteins at ordinary ambient temperatures and may cause severe chemical burns. It is highly soluble in water, and readily absorbs moisture and carbon dioxide from the air. It forms a series of hydrates \[NaOH\cdot n{{H}_{2}}O\] . The monohydrate $NaOH\cdot {{H}_{2}}O~$ crystallizes from water solutions between \[12.3\text{ }-\text{ }61.8~{}^\circ C\] .
Sodium hydroxide is a deliquescent solid. When it comes in contact with air, it absorbs moisture and carbon dioxide from air to form sodium carbonate. Hence, the standard solution of sodium hydroxide cannot be prepared by direct weighing of cold NaOH. It is a secondary standard substance.
So, the correct answer is “Option C”.

Note: A primary standard is a reagent that is extremely pure, stable, has no waters of hydration and has high molecular weight. Some primary standards of titration of acids include sodium carbonate. Primary standards are used to prepare standard solutions for titrations.