What is a SequenceFile?

A.
A SequenceFile contains a binary encoding of an arbitrary number of homogeneous Writable
objects

B.
A SequenceFile contains a binary encoding of an arbitrary number of heterogeneous Writable
objects

C.
A SequenceFile contains a binary encoding of an arbitrary number of WritableComparable
objects, in sorted order.

D.
A SequenceFile contains a binary encoding of an arbitrary number key-value pairs. Each key
must be the same type. Each value must be the same type.

Explanation:
SequenceFile is a flat file consisting of binary key/value pairs.
There are 3 different SequenceFile formats:
Uncompressed key/value records.
Record compressed key/value records – only ‘values’ are compressed here.
Block compressed key/value records – both keys and values are collected in ‘blocks’ separately
and compressed. The size of the ‘block’ is configurable.
Reference: http://wiki.apache.org/hadoop/SequenceFile