(a): #(S), (S)#, (b): #(R), (S)# and (c): #(S), (S)#
Explanation:
The configurations of each stereocentre is depicted in the scheme below:
(a):
1- Assign priorities by decreasing atomic number. For example, #color(blue)(""_8O)#, #color(red)(""_35Br# and #color(green)(""_1H#, the priority is #color(blue)(2), color(red)(1) and color(green)(3)# respectively.
2- For both stereocentres the rotation is counter clockwise; therefore, their configurations are #color(blue)((S)) and color(red)((S))#.
(b):
1- Assign priorities as explained in (a).
2- #color(blue)("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 #color(blue)((R))#.
3- #color(red)("For the red stereocentre"# The rotation is counter clockwise with the lowest priority group facing backward, then the configuration is #color(red)((S))#.
(c):
1- Assign priorities as explained in (a).
2- #color(blue)("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 #color(blue)((S))#.
3-#color(red)("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 #color(red)((S))#.
Here are two videos that explain in details how to label stereogenic centres R or S:
