I'll give a medal.
Which word group best completes the sentence without introducing an inappropriate shift in tense?

He had been reading a book

A.

when the thunderstorm is starting.

B.

when the thunderstorm will start.

C.
when the thunderstorm starts.

D.
when the thunderstorm started.

Question

I'll give a medal. 
Which word group best completes the sentence without introducing an inappropriate shift in tense? 

He had been reading a book
  
 
 

 
 

  

  A. 
 
when the thunderstorm is starting.
   
   

 
 

  

  B. 
 
when the thunderstorm will start.
   
   

 
 

  

  C. 
  when the thunderstorm starts.   
   

 
 

  

  D. 
  when the thunderstorm started.

anonymous · Answer

Answer: D- When the thunderstorm started.
The original sentence "He had been reading a book" is of past tense. 
A and C are in present tense while B is in future tense. 
The only answer left is D which is also a past tense, therefore it doesn't introduce any inappropriate shift in tense.