Theory:

     Duttada looked at her. Did she know? How could she? He had never mentioned Project Light Brigade to her. He probed cautiously, “I don’t understand what you mean.”
 
     “It is very simple. There were no disasters because of the yajna at our house.”
 
     “But I never performed the yajna. Don’t you remember, I refused to have anything to do with it?”
 
     “Of course, I do. But we found a way out — at least Guruji did. He said that if you were unwilling to perform the yajna, it would be all right if a descendant of yours did it. So we got Khoka to deputise for you. And it has worked! Isn’t Guruji clever?” Indrani’s voice had a ring of triumph.
Explanation:
 
Duttada was so taken aback that he glanced at his wife in astonishment. He suspected whether his wife was aware of Project Light Brigade. He knew that he did not tell her anything about it. Even when the Project (Light Brigade) was completed, he kept the project secret to himself and did not share it with his wife.
 
Duttada carefully responded back to his wife's inquiries, pretending to be ignorant of her question, and she informed Duttada that Comet Dutta had passed by without damaging the Earth as a result of the yajna they had performed at Duttada's residence. Duttada was relieved in one way after hearing her wife's response, as he was comforted to learn that his wife knew nothing about the Project Light Brigade. Still, he couldn't understand how his wife was referring to unperformed yajna, because as he knew, he didn't perform any yajna at home. He had refused to take part in it as well, considering those rituals as superstitious.
 
Indrani Debi revealed the secret to her husband following his question, saying, yajna was performed on that particular day at Duttada's home by their grandson Khoka as per Guruji's advice. According to Guruji, a descendant of Duttada could perform yajna on behalf of Duttada. So, since Duttada refused to perform yajna at home, Indrani made her grandson to do it on behalf of him. Indrani also praised Guruji as a clever person since he had come up with a idea that had worked well because, according to her, such an idea had caused Comet Dutta to pass Earth without harming anybody. Her voice had a strong tone of accomplishment about it.
 
Meaning of difficult words:
  
S.No
Words
Meaning
1.
Cautious
Being careful in order to avoid danger
2.
Triumph
A great success
3.
Deputise To act as a substitute
Reference:
National Council of Educational Research and Training (2008). The Comet - II — JAYANT NARLIKAR (pp. 81-87). Published at the Publication Division by the Secretary, National Council of Educational Research and Training, Sri Aurobindo Marg, New Delhi.