Final answer:
The last date the product should be used by is 12/31/99.
Step-by-step explanation:
The product is labeled to expire 1299, so the date format should match the label. Among the options provided, a) 12/1/99, b) 1/1/00, c) 11/31/99, and d) 12/31/99, the correct date format in month/day/year is d) 12/31/99. This is the last date the product should be used by.