‘Q’ will be the first part of the given sequence as it gives the way inside the houses made of baked bricks, that is, upstairs rooms were accessed by a narrow stone staircase at the back of the house. ‘P’ will follow next as ‘this staircase’ refers to the ‘upper staircase’ in ‘Q’. ‘S’ will come next because it further gives the description of the house. ‘R’ will follow next as it makes a reference to ‘drains’ in relation of ‘drainage systems’ mentioned in ‘S’. Hence, QPSR is the correct sequence. Thus, option (d) is correct.