Question

because, but, & so - american sign language
historically, american sign language (asl) has mainly been used by deaf people and people who are hard of hearing, the population of hearing people who know asl has been growing.
because but so ← joining words
screenreader users: make sure to read through the whole sentence before filling in the blanks.
fill in the blank with the correct joining word.

Explanation:

The two clauses present a contrast: the first states ASL was historically used mainly by deaf/hard of hearing people, while the second notes hearing ASL learners are growing. "But" is the conjunction used to show this contrast.

Answer:

but