The correct answer is option 'C': multiple.

Explanation:
To understand why the correct answer is 'multiple', let's first define what a multiple is. A multiple of a number is the result of multiplying that number by an integer (whole number). For example, the multiples of 3 are 3, 6, 9, 12, 15, and so on.

Now, let's consider the given statement: "Every ______ of a number is greater than or equal to that number." This statement implies that whatever fills the blank, it should be something that, when multiplied by a number, the result is greater than or equal to the original number.

Let's consider the options provided:
a) Factor: A factor of a number is a whole number that divides the given number without leaving a remainder. For example, the factors of 12 are 1, 2, 3, 4, 6, and 12. However, if we multiply a number by one of its factors, the result may or may not be greater than the original number. For instance, if we multiply 12 by 2, we get 24, which is greater than 12, but if we multiply 12 by 3, we get 36, which is greater than 12. Therefore, a factor is not always greater than or equal to the original number.

b) Number: This option is incorrect because it is too broad. The statement is already referring to a number, so it doesn't provide any additional information.

c) Multiple: A multiple of a number, as mentioned earlier, is obtained by multiplying the number by an integer. Since we are multiplying the number, the resulting multiple will always be greater than or equal to the original number. For example, if we multiply 5 by 2, we get 10, which is greater than 5. Similarly, if we multiply 5 by 1, we get 5, which is equal to the original number. Therefore, every multiple of a number is greater than or equal to that number.

d) None of these: This option is incorrect because the correct answer is 'C' - multiple.

In conclusion, the correct answer is 'C' - multiple because every multiple of a number is greater than or equal to that number.