‘S’ will be the first part of the given sequence as it is in contrast with the given sentence as it tells us about what happened next which is clearly indicated from the words, ‘His invention was used later by other clever men to give us the railway engine’. ‘Q’ will come next as it is in continuation with the previous statement, that is clear from the word, ‘then’, that is, ‘Then many years later, the petrol engine was invented’. ‘P’ will come next as it is in continuation with the previous statements which is indicated from the words, ‘With petrol engines people were able to build motor cars and aeroplanes'. ‘R’ will follow logically as it is in continuation to the previous statement which is indicated from the word, ‘These’. Hence, SQPR is the correct sequence. Thus, option (a) is correct.