Multiplication and division became easier during the Gupta Empire primarily because of the advancements in mathematics that took place during that time, including the development of the decimal place value system and the concept of zero. These innovations made calculations more straightforward and efficient.
Therefore, the correct reason among the given options would relate to the creation of a numeral system that included zero, which revolutionized mathematical computations.
So, a more accurate response would be: "They created a numeral system that included zero."