At the 2016 Democratic National Convention, emerged as a powerful and heavily criticized force in the nominating process because of the closeness of the race between Bernie Sanders and Hillary Clinton.

superdelegates

union leaders

unpledged delegates

women's group leaders

special interest group leaders

The correct response from the options provided is superdelegates.

Explanation:

There were a number of controversies at the 2016 Democratic National Convention.  One involved the superdelegates and the other the Convention Chair.  There were leaked emails about the Democratic Convention Commitee that showed evidence that  the staffers favored Clinton. This led to the withdrawl of chairperson Debbie Wasserman Schultz who was not allowed to speak at the convention. Sanders was also critical of the superdelegate system and pressed for reform.  The conpromise that was reached was that two-thirds of superdelegates would be bound to the results of state primaries and caucuses. Only one third comprised current national democratic representatives and governors would be allowed to support the candidates of their choice at the convention.  Up until and including the convention in 2016, the superdelegates were not obliged to vote the same way as their state's primary election or caucus.