Hi Anna, 
That's a good question. In that one, the salesperson presents the argument's main conclusion in the first sentence of the stimulus (to be a successful salesperson you need at least three years in sales). Let's reorder this stimulus, and begin by considering the premises of the argument: 
Premise i)  To succeed as a salesperson, you need to establish a client base: 
 
 
  
 succeed in sales   

   establish client base
Premise ii)  According to studies, if you spend 
at least 3 years developing a client base, you can make it in sales. Note the new condition that has now appeared--
at least 3 years:
 
 
  
 3+ years establishing client base  

 comfortable living in sales
Note that this condition is distinguishable from the necessary condition in the first premise, which requires that a client base be 
established but doesn't specify a minimum number of years. 
Conclusion:  When a salesperson is successful, it shows that the person has been in sales for 3+ years: 
 
 
  
 Succeed in sales  
 3+ years
   3+ years in sales
Why is this conclusion flawed?  It is not supported by either premise. 
Premise (ii) provides that if you spend 3+ years you can make a comfortable living in sales:
 
 
  
 3+ years establishing client base  

 comfortable living in sales
This does not support the conclusion, which, at best, reverses this relationship:
 
 
  
 succeed in sales   

   3+ years in sales
(I say "at best" because it is not entirely clear that making a comfortable living is the same as success)
As for Premise (i), it provides that to succeed in sales you need to establish a client base. This is very different from the conclusion's more specific requirement that to succeed in sales you need to put in at least three years: 
Premise:  
 
   
   
  succeed as a salesperson   

   establish client base
(Flawed) Conclusion:  

 succeed as a salesperson   
 at least 3 years
  at least 3 years in sales
Based on the premise above, we know that if you are a successful salesperson you need to establish a client base. The problem is that that doesn't necessarily require 3+ years.   
Again, the premise merely requires that a client base be 
established. The author jumps to the conclusion that salesperson success requires at least three years. This is what correct answer choice (B) provides: the author doesn't consider the possibility that a client base could be established in under three years. 
Tricky one!  I hope that's helpful--please let me know whether this is clear--thanks!
~Steve