What data does a Reducer reduce method process?

A.
All the data in a single input file.

B.
All data produced by a single mapper.

C.
All data for a given key, regardless of which mapper(s) produced it.

D.
All data for a given value, regardless of which mapper(s) produced it.

Explanation:
Reducing lets you aggregate values together. A reducer function receives an iterator
of input values from an input list. It then combines these values together, returning a single output
value.
All values with the same key are presented to a single reduce task.
Reference: Yahoo! Hadoop Tutorial, Module 4: MapReduce