Question

A pluralistic society guards against one group taking over by

Answer 1

A pluralistic society is a society where there are many social groups, and where none of the groups is dominant over the other groups. 

So all groups work together so that not one of them takes the power over the other groups by force. ("by force" is one possible answer).

Answer 2

I believe the answer is: having the constant mix of diverse groups enter into politics

In a pluralistic society, the power within a society is divided into more than one social groups/ethnicity. This mean that during the election, each social groups could voted for a representative from their own group who would fight for the aspiration of the group within the government body.