‘S’ will be the first part of the given sequence as it introduces the subject and the verb, that is, ‘the plane couldn’t take off’. ‘P’ will come next as it tells us till what time the action could not take place, that is, ‘for an hour’. ‘Q’ will follow next as it begins with the word ‘because’ and in combination with ‘R’ tells the reason for the delay in the flight. So, SPQR becomes the right sequence of words. Hence, option (d) is correct.