i think structures of domination are intersecting and that it is pointless to find the "ultimate" source of oppression. While eliminating capitalism may address inequities in class, it will not purge western culture of its racisms and sexism. Race, gender and class all function as relatively independant, yet connected structures of domination, they all must be dismantled. one instance that illustrates how confouding these structures can be was the OJ simpson case. What was the key factor in that case, was it that OJ had the money to buy the best legal team in the world(class), or was it that OJ was a black man being set up by the police(race) or was it that OJ was a man that beat his wife(gender), or was it all of these factors in a mangled knott? Advocates for each position vehemently argued their case, but imagine eliminating one factor, such as class, would not the problem persist?
i think folks like to reduce the struggle to a single factor like, "its all class" or "its all race", because including one of these contesting elements could potentially indict them. Folks like to be righteous but they dont like to be culpable.
but to answer your question: a. race b. class c. gender/sexuality d. all of the above