select the sentence with the misplaced modifier.
the group of teens agreed that they’d meet at the mall after school. heading up the escalator, they spotted many shops that interested them. one shop had a great sale on summer clothes, and the teens loaded up their arms with outfits to purchase. the salesperson handed a receipt to each customer that was very long.

Answer.  The final sentence:

"The salesperson handed a receipt to each customer that was very long. "
 
   →  is the sentence that contains a misplaced modifier.

    → Specifically, the placement of the phrase: "that was very long".

This is a misplaced modifier, because it is difficult to determine if "very long" modifies either: 1)  the actual "receipt" ; or, 2) the "customer(s)" —specifically, "each customer [that happened to be 'very long"].