For each intermediate key, each reducer task can emit:

For each intermediate key, each reducer task can emit:

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

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

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

E.
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)

Explanation:
Reducer reduces a set of intermediate values which share a key to a smaller set of
values.
Reference:Hadoop Map-Reduce Tutorial