Non-manual markers in sign languages are most fittingly categorized as adverbs, given their role in modifying meaning and conveying nuances in signed communication. Therefore, option 4 is correct.
4) "Adverb" is the best option. Non-manual markers in sign languages frequently work adverbially, changing the meaning of signed utterances by adding context, emphasizing a point, or Signaling the speaker's mood.
Although non-manual indicators do not cleanly fit with traditional parts of speech, classifying them as adverbs provides a pragmatic approximation within the context of standard grammatical categories.
This option acknowledges the dynamic and diverse nature of non-manual indicators while emphasizing their proclivity to change and enhance the overall meaning of signed communication in a way similar to adverbs.