The correct option is D One evening when we heard fire engines rushing past my house
We need to choose an option that describes an event that took place after P and before R.
From the options, B contains the words "when we heard fire engines...", sentence R seems to complete this sentence in terms of time 'when': "when we heard fire engines,... we rushed out and saw...". Hence option B is correct.
Option A cannot come before sentence R, after 'joining a crowd outside' you cannot 'rush out', the latter phrase comes first. Hence A is incorrect.
Similarly, option C is a general statement, whereas the sentence R specifies an event, which much be preceded by a sentence that can complete the meaning logically. Hence C is also incorrect.
Option D is not related to the sentences, (if you place it before R: 'we went out', then 'we rushed out', are two different events that cannot precede one another) hence D is incorrect.