Here is the item again:
Because of all his responsibilities at work and
school, (A) Michael had
no time to devote to the yard, (B)
so weeds, vines, and insects thrived in this
jungle. (C)
That neighbors looked on with disapproval.
You correctly realized that the period after jungle created a fragment. That neighbors looked on with disapproval is a subordinate clause, so it cannot stand alone as a complete sentence.
To fix this fragment, you chose to do this:
Because of all his responsibilities at work and school, Michael had no time
to devote to the yard, so weeds, vines, and insects thrived in this
jungle that neighbors looked on with disapproval.
You have correctly employed Punctuation Rule 1:
Main clause + Ø + subordinate clause.
Let the right answers ring!