Question

because, but, & so - american sign language
there are hundreds of varieties of sign language, people who use
different varieties of sign language may not be able to understand each
other.
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 first clause states there are many sign language varieties, and the second clause states a resulting consequence (users of different varieties can't understand each other). "So" is used to show a cause-and-effect relationship where the first part leads to the second.

Answer:

so