Final answer:
A factor is a whole number that divides another number without leaving a remainder, as referenced by the factors of 18 which include 1, 2, 3, 6, 9, and 18.
Step-by-step explanation:
A factor is option A: A whole number that divides another number without leaving a remainder. When we talk about the factors of 18 which are 1, 2, 3, 6, 9, and 18, this means that each of these whole numbers can be divided into 18 evenly, resulting in a whole number quotient with no remainder. Therefore, a factor is not a decimal number, not a number that cannot be divided by any other number, and is certainly not a number that leaves a remainder when divided by another number.
For example, if we take the number 6 as a factor of 18, dividing 18 by 6 results in 3, which means 6 evenly divides into 18. This concept is different from needing to perform calculations with decimals or dealing with significant figures, as factors must always be whole numbers.