Complete Question 
The complete question is shown on the first uploaded image 
Answer:
X  is Be
Y  is B 
Z  is  O            
Explanation:
In order to understand the solution, we need to that the an element form a bond based on 
 1  The number of valence electron available in its atom 
 2 The  number of electron it require to attains its stable state i.e its Octet  state 
         The number of valence electron for each of the option  are 
                  F = 7,   N = 5 , O = 6 , C = 4 , B = 3 , Be = 2
  Considering the first element 
            We see that  X is donating  two valence electron in its atom and it is donating it to 2  Cl as Cl  only requires a single electron to achieve its stable state 
Now looking  option we can see that  Be has two valence  electron in it atom and in order for it to attain its stable state it needs to release the electron so the correct option for this question is Be
 
Considering the second  element               
       We see that  Y is donating  three valence electron in its atom and it is donating it to 3  Cl as Cl  only requires a single electron to achieve its stable state     
     Now looking  option we can see that  B has three valence  electron in it atom and in order for it to attain its stable state it needs to release the valence electron so the correct option for this question is B 
Considering the second  element  
       We see that  Z is donating two valence electron in its atom and it is donating it to 2  Cl as Cl  only requires a single electron to achieve its stable state, also Z has 2 lone-pair valence electron 
        Now looking at the option we can see that  O has 6 valence electron so if it donates 2 valence electron it will still be left with 4 - pair of electron
    Therefore the correct option to this question is  O