Fractions can be added once they have what?

Fractions can be added when they have a common denominator because this allows for the fractions to represent parts of the same whole. When two or more fractions share the same denominator, you can combine the numerators directly while keeping the denominator unchanged. This process ensures accuracy in the addition, as the fractions are now comparable.

For example, if you are adding 1/4 and 1/4, both fractions have a denominator of 4, so you can simply add the numerators (1 + 1) to get 2, resulting in 2/4, which can be simplified to 1/2. Without a common denominator, you would be attempting to add parts that represent different sizes or segments of a whole, which would lead to incorrect results.

The other options refer to factors or numerators without addressing the necessity of matching denominators, which is not sufficient for properly performing fractional addition.