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