Questions

The phrase ‘that he could not serve’ is grammatically correct. This is because other forms of verb doesn’t go with the blank and this form of verb is grammatically and contextually correct. Thus, the correct formation would be, ‘He was too conscientious in the discharge of his duties to serve that exploiter for long’.

‘To dispose of’ is the correct phrase to be used here. It means to get rid of something by selling or giving. Thus, the correct formation would be, ‘Raman wants to dispose of his house’.

From all the hints given, 

only 042 satisfies and it unlocks the key.