‘S’ will be the first part of the given sequence as per the subjectverb agreement rule, that is, “could not attend”. ‘R’ will come next as it is the object on which the action is performed, that is, ' on Sheila’. ‘P’ will come next as it starts with the word, ‘when’ which tells us about the time at which the action was performed and ‘Q’ will follow logically to complete the sequence. Hence, SRPQ becomes the right sequence of words. So, option (b) is correct