Why is this stereocenter by the Br an R? I thought it was S?
Both structure (a) and structure (b) have misassigned chirality centers. There should be only one $R$ assignment over all four structures. The correct assignments are as follows:




  Structure (a)


The misassigned chirality center is the one in the center. The carbon atom with the OH group has the highest priority (one C-O bond beats any number of C-C or C-N bonds). The second highest priority is the pyrrole ring (one C-N bond and one C=C bond beats one C-N, one C-C, and one C-H). The carbon atom with the primary amine is the third priority. The groups are arranged counter-clockwise and the lowest priority (H) is in the back. 




  Structure (b)


The misassigned chirality center is the one with the bromo group. The bromine atom is the highest priority. The carbon atom bonded to chlorine is second, and the carbon atom bonded to fluorine is third. The groups are arranged clockwise, but the lowest priority (H) is forward.