The word ‘sustained’ (Adjective) means : uninterrupted; to make something continue for sometime without becoming less. Look at its use in the sentence : China has seen a period of sustained economic growth in the recent past. Out of the given alternatives the word ‘steady’ (Adjective) means : fixed, uniform, regular. , Look at its use in the sentences : We are making slow but steady progress. The castle receives a steady, stream of visitors. Hence, the words sustained and steady are synonymous