Charm is a kind of appeal or attactiveness, and its both presence or lack are discounted here. So, focus is entirely on the part of the sentence following a blank. To be deterrent means to set off or call quit by enforcing action or simply intimidating. The right pair is minatory and menacing. Correct answers are B, C.
Note: check economist's vocab list for easy difficulty words 
https://gre.economist.com/gre-advice/gre-vocabulary/which-words-study/most-common-gre-vocabulary-list-organized-difficultyCarcass wrote:
Chosen not as much for his charm, or lack thereof, as for his__________appearance, the singer’s bodyguard proved to be an effective deterrent for many autograph-seekers.
  
    | Two Answers | 
     
  
    | A. placid | 
    
  
  
    | B. minatory | 
   
  
  
    | C. menacing | 
    
  
D. gargantuan | 
    
E. seedy | 
    
    | F. livid |