A. in the late 1980s and early 1990s had either followed a standard career path, progressing from business analysts to directors, or were
'had' before the beginning of the idiom 'Either X, Or Y' means --had either followed a standard career path.. OR had were accounting professionals. This seems nonsensical, so eliminate A
B. in the late 1980s and early 1990s either followed standard career paths, ranging from business analysts to directors, or had been
the first part of the idiom is in simple past whereas in the second part of the idiom(or else had been) is in past-perfect, making it seem like that action happened before, which is not correct. Usage of 'had' is not required as both the actions took place in the same time range.So, eliminate B.
C. of the late 1980s and early 1990s either had followed a standard career path, progressing from business analyst to director, or were
The usage of had is acceptable here,since they first followed a carrer path, which later progressed
D. in the late 1980s and early 1990s had followed standard career paths, and had progressed from business analysts to directors, or else were
usage of had twice in the first part of the idiom violates the usage of past participles w.r.t. time. So, eliminate D.
E. of the late 1980s and early 1990s followed a standard career path, ranging from business analyst to director, or else had been
the correct idiom is Either X, Or Y. But here, its Either X, or else Y--which seems incorrect.
the first part of the idiom is in simple past whereas the second part(or else had been) is in past-perfect, making it seem like that action happened before, which is not correct. So, eliminate E.
Seems like we're left with C
Hence, C is the correct option