ANS:C - changing the cardinal points, i.e. substituting N for S and E for W and vice-versa

In WCB, back bearing is obtained by BB=FB + or - 180, where subtraction is done when FB is more and addition is done when FB is less than 180.

In Reduced bearing system, or Quadrantal bearing system the numerical are same but the letters are put opposite. The horizontal angle made by a line with the magnetic north or south (whichever is closer from the line) in the eastward or westward direction is the Quadrantal Bearing or Reduced Bearing of the line.

SO option C (changing the cardinal points, i.e. substituting N for S and E for W and vice-versa) is right For whole circle Bearing:

BB = FB +-180.
BB = FB +180 (LESS THAN 180).
BB = FB -180 ( MORE THAN 180).

FOR QB SYSTEM:

BB = FB (BUT ONLY DIRECTION CHANGE).
N - S.
S - N.
E - W.
W - E. The horizontal angle made by a line with the magnetic north or south (whichever is closer from the line) in the eastward or westward direction is theQuadrantal Bearing or Reduced Bearing of the line.

In quadrantal bearing or reduced bearing, both north and south are considered as reference meridians. Depending upon the position of a survey line, the direction of the reference meridian to the line can be either clockwise or anticlockwise. In the expression of the reduced bearing value of a line, quadrant has to be mentioned in which the line lies. In this question, bearing is taken at QB system. Between fore and back bearing the difference is 180'. In QB system certain if we add (when the angle is <90) or subtract 180(when the angle is >90) from a certain angle then the resultant angle can find out by changing N to S and E to W. It is quadrant bearing system not whole circle so for back bearing we just have to change the quadrant just opposite of given not sub or add 180.