The balance in the Prepaid Rent account before adjustment at the end of the year is $$19200,$ which represents three months' rent paid

on December $1.$ The adjusting entry required on December $31$ is to

debit Rent Expense, $$12800$; credit Prepaid Rent $$12800.$

debit Prepaid Rent, $$12800$; credit Rent Expense, $$12800.$

debit Rent Expense, $$6400$; credit Prepaid Rent, $$6400.$

debit Prepaid Rent, $$6400$; credit Rent Expense, $$6400.$