Answer:
They most have common denominators.
Step-by-step explanation:
Let the following two rational expressions:

where p,a,q, b are integers, a and b the denominators are not 0, i.e. 
We can add rational expressions only if their denominator is same.
That is why we find LCD to be
.
Then,

Let the following two rational expressions:

where p,a,q, b are integers, a and b the denominators are not 0, i.e. 
We can add rational expressions only if their denominator is same.
That is why we find LCD to be
.
Then,

So, the correct answer is the last option that we can sum rational expressions if they have common denominator.