Here is the item again:
As Alex squeezed down the narrow row of chairs.
(A) His heavy book bag
bumped the large soda perched on a desk,
(B) sending a shower of diet
cola into Teresa's lap, (C)
soaking her jeans.
You correctly realized that the period after chairs created a fragment. As Alex squeezed down the narrow row of chairs is a subordinate clause, so it cannot stand alone as a complete sentence.
To fix the problem, you chose to do this:
As Alex squeezed down the narrow row of chairs,
his heavy book bag bumped the large soda perched on a desk, sending
a shower of diet cola into Teresa's lap, soaking her jeans.
You have correctly employed Punctuation Rule 2:
Subordinate clause + , + main clause.
Good answer!