Theory:

     ‘I can’t see anything,’ said Suresh.

     ‘It’s just the top of the plum tree,’ said Binya.

     They were on the path leading across the saddle of the hill.

     ‘Goodbye, goodbye...’

     Voices on the wind.

     ‘Who said goodbye?’ asked Usha.

     ‘Not I,’ said Suresh.

     ‘Not I,’ said Binya.

     ‘I heard someone calling.’

     ‘It’s only the wind.’

     Usha looked back at the ruins. The sun had come up and was touching the top of the walls. The leaves of the plum tree shone. The thrush sat there, singing.

     ‘Come on,’ said Suresh. ‘I’m hungry.’

     ‘Goodbye, goodbye, goodbye, goodbye....’

     Usha heard them calling. Or was it just the wind?
Explanation:
 
Suresh said that he could not see anything like that.
 
Binya cleared that it is just the top of the plum tree.
 
They went ahead till the end of the hill.
 
Usha heard someone saying, "Goodbye, goodbye!"
 
Usha asked who said goodbye.
 
Both Suresh and Binya said they did not utter a word.
 
Usha said she heard someone calling her.
 
It might be just the wind, said Binya.
 
Usha looked back again at the ruins, the sun had come up and was on the top of the walls of the ruins, the plum tree stood, as usual, the thrush was still singing.
 
Suresh got irritated and said he was hungry.
 
Usha could still hear the sound of someone saying goodbye to her, or maybe it was just a wind.
Reference:
State Council of Educational Research and Training (2019). Term-1 English Standard-7. The Wind on Haunted Hill by Ruskin Bond  (pp. 25-34). Published by the Tamil Nadu Textbook and Educational Services Corporation.