Here's why:
* It names a feeling or attitude: "Disapproval" refers to the feeling of not approving of something.
* It can be the object of a verb: "She expressed her disapproval of the plan."
* It can be modified by adjectives: "Her disapproval was palpable."
However, "disapproval" can also be used as a verb in some cases, meaning "to disapprove of something". This usage is less common, but it's grammatically correct.
For example:
* "The committee disapproved the proposed budget."
So, the part of speech for "disapproval" depends on how it's used in a sentence.