For this case, we have that by definition, the difference is represented by the mathematical sign "-", also called signs of subtraction or sign of subtraction. 
Then we discard options A and B. 
We have the option C: 

It contains a sign of addition and also refers to three terms. 
Finally, in the last option, we have a difference of two terms. 
 is the first term
 is the first term 
 is the second term
is the second term 
ANswer: 
Option D