Answer: 
 
 
Explanation:
 The missing options are:
 

 For this exercise it is important to remember the mulitplication of signs:
 

 By definition, the sum is the result of an addition.
 Therefore, you need to add the numbers given in the exercise (-15 and 6) in order to find its sum. This is:
 
 
 
 (Since the greatest number has a negative sign, the sum is negative).
 Knowing this, you can check each option to find the expression whose sum is -9:
 

 The answer is the option "d".