Answer:
x+3
Step-by-step explanation:
If you factor 5 out of the denominator of the first expression, 5x+15 turns into 5(x+3).
Then, since you have x+3 in the numerator and denominator, it will cancel out.
The steps are shown below:

Ive attached an image that shows an example of simplifying an expression using a the same technique.
If you do the problem in the order of operations (PEMDAS), it should be B. 18