A.  The Hadoop administrator has to set the number of the reducer slot to zero on all slave nodes. This will disable the reduce step.
B.  It is imposible to disable the reduce step since it is critical part of the Mep-Reduce abstraction.
C.  A developer can always set the number of the reducers to zero. That will completely disable the reduce step.
D.  While you cannot completely disable reducers you can set output to one. There needs to be at least one reduce step in Map-Reduce abstraction.

Answer: C