Answer: 
The correct option is A. 
Step-by-step explanation: 
If the degree and variables of two terms are same, then they are called like terms. 
In option A. 
The given expression is 
 
 
Here, 4ab and -3ab are like terms because both have same variables a and b with degree 1. 
By combining like terms we get 
 
 
 
 
In options B, C and D, there are no like terms, So only the expression 
 we can simplify by combining like terms.
 we can simplify by combining like terms. 
Therefore the correct option is A.