The principal reason that solid sodium hydroxide is not used as a primary standard for acid-base titrations is that it:

A primary standard is an insoluble solid chemical with a large molar mass that is exceedingly transparent, has a constant formula, and does not change when exposed to the atmosphere.

should be appropriate for use as a primary standard:

1) a compound must first be readily available and inexpensive.

2) be very pure

3) have a known formula

4)be an unaltered in air during weighing, that is, the compound must not

 a) absorb moisture from the air.

 b) absorb carbon dioxide from the air

c) be oxidized by air

d) have a relatively large molar mass to minimize errors during weighing.

$\mathrm{NaOH}$ quickly absorbs water from the atmosphere, making it unsuitable for use as a main standard. They easily absorb atmospheric carbon dioxide.

Hence, option A is correct.