Part A:
The BEST explanation for the source of the speaker's social media lows is:
B. self-doubt
Part B:
The lines from the text that BEST support the answer to part A are:
B. And I wonder if I’ve said something wrong / or if I alone lack purpose beyond my screen.
These lines express the speaker's self-reflection and uncertainty about their worth and purpose in relation to social media interactions, highlighting feelings of self-doubt.