Answer:
B. (x-3)
Explanation:
 
Factorise the equation with suitable values. 
In order to get 6 from x²-5x+6, 
as an example, use the numbers 3 and 2.
We will get to factorise it to become (x-3)(x-2)
Based on the choice answers given, the answer is B. (x-3)