The configurations of each stereocentre is depicted in the scheme below:
(a):
1- Assign priorities by decreasing atomic number. For example, 8O, 35Br and 1H, the priority is 2,1and3 respectively.
2- For both stereocentres the rotation is counter clockwise; therefore, their configurations are (S)and(S).
(b):
1- Assign priorities as explained in (a).
2- For the blue stereocentre The hydrogen with priority 4 switch place with COOH with priority 2. The rotation will be counter clockwise which means (S), but because of switching places between 2 and 4 the configuration becomes (R).
3- For the red stereocentre The rotation is counter clockwise with the lowest priority group facing backward, then the configuration is (S).
(c):
1- Assign priorities as explained in (a).
2- For the blue stereocentre The lowest priority H is in the plane and the highest priority Br is facing backward. These groups should switch positions and the rotation will be clockwise which means (R), but because of switching places between 1 and 4 the configuration becomes (S).
3-For the red stereocentre The lowest priority H is in the plane and the highest priority OH is facing backward. These groups should switch positions and the rotation will be clockwise which means (R), but because of switching places between 1 and 4 the configuration becomes (S).
Here are two videos that explain in details how to label stereogenic centres R or S:
