Option A: 'Interest' refers to the 'money paid for delaying the payment of a debt'. Here it refers to the money that must be paid in addition to the 20,000 rupees as the payment of taxes has been delayed.
Hence this word completes the sentence in meaning and option A is correct.
Option B: 'Taxes' refers to 'a compulsory contribution to the state revenue levied by the government on a citizen's income, or on some goods and services'. The taxes here are 20,000, as already mentioned before. Hence this term isn't used again.
Option C: 'Principal' denotes an 'original sum of money invested or lent'.
Option D: 'Returns' refers to the profits made on an investment.
These words are not relevant in meaning and context. Hence options B, C and D are wrong.