Under Presidential Decree No. 851, the 13th month pay must be at least one-twelfth of the total basic salary earned within the calendar year and must be paid not later than:

The correct answer is A. 24 December

PD 851 of 1975 requires employers to pay rank-and-file employees a 13th month pay of at least one-twelfth of their total basic salary for the year, not later than 24 December. Government employees receive a year-end bonus under separate rules.