Indian Institute of Foreign Trade 2012 Solved Paper

'Indefatigable' means 'tireless' whereas 'inveterate' means 'having a particular habit, activity, or interest that is long-established and unlikely to change' i.e 'habitual'
Thus, option A and B can be eliminated.
Between option C and D. Impoverished in D means 'make (a person or area) poor' which is different from tireless.
Thus, option C is the correct answer.