Answer:
February 6, invoice is paid:
Dr Accounts payable 1,000
  Cr Cash 1,000
Explanation:
January 9, merchandise is purchased on account:
Dr Merchandise inventory 1,500
  Cr Accounts payable 1,500
January 10, freight charges are paid:
Dr Merchandise inventory 120
  Cr Cash 120
January 14, partial return of merchandise:
Dr Accounts payable 500
  Cr Merchandise inventory 500
February 6, invoice is paid:
Dr Accounts payable 1,000
  Cr Cash 1,000