Which of the following numbers is a common factor of 24 and 36?

Factors of a number divide the number completely. without leaving any remainder. 

For example, 2 is a factor of 6 because $6 ÷ 2 = 3$ and the remainder of the division is zero. 

Similarly, when 6 is divided by 3, $6 ÷3=2$ and the remainder is zero. 

We can rearrange the above equation as $6= 2 \times 3$. We can see that the number can be written as the product of its factors.

So, Factors of 24 = 2 x 2 x 2 x 3 

and Factors of 36 = 2 x 2 x 3 x 3 

Now, the highest common factor of 24 and 36 can be found out as: 

2 x 2 x 3 = 12.

Here only 12 is mentioned in the options, therefore it is the correct answer.