For each intermediate key, each reducer task can emit:

A.
As many final key-value pairs as desired. There are no restrictions on the types of those keyvalue pairs (i.e., they can be heterogeneous).

B.
As many final key-value pairs as desired, but they must have the same type as the intermediate
key-value pairs.

C.
As many final key-value pairs as desired, as long as all the keys have the same type and all the
values have the same type.

D.
One final key-value pair per value associated with the key; no restrictions on the type.

E.
One final key-value pair per key; no restrictions on the type.

Explanation:
Reducer reduces a set of intermediate values which share a key to a smaller set of
values.
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.
Reference: Hadoop Map-Reduce Tutorial; Yahoo! Hadoop Tutorial, Module 4: MapReduce