A.  Map files are stored on the namenode and capture the metadata for all blocks on a particular rack. This is how Hadoop is “rack aware”
B.  Map files are the files that show how the data is distributed in the Hadoop cluster.
C.  Map files are generated by Map-Reduce after the reduce step. They show the task distribution during job execution
D.  Map files are sorted sequence files that also have an index. The index allows fast data look up.

Answer: D