On a cluster running MapReduce v1 (MRv1), a TaskTracker heartbeats into the JobTracker on
your cluster, and alerts the JobTracker it has an open map task slot.
What determines how the JobTracker assigns each map task to a TaskTracker?

A.
The amount of RAM installed on the TaskTracker node.

B.
The amount of free disk space on the TaskTracker node.

C.
The number and speed of CPU cores on the TaskTracker node.

D.
The average system load on the TaskTracker node over the past fifteen (15) minutes.

E.
The location of the InsputSplit to be processed in relation to the location of the node.

Explanation:
The TaskTrackers send out heartbeat messages to the JobTracker, usually every
few minutes, to reassure the JobTracker that it is still alive. These message also inform the
JobTracker of the number of available slots, so the JobTracker can stay up to date with where in
the cluster work can be delegated. When the JobTracker tries to find somewhere to schedule a
task within the MapReduce operations, it first looks for an empty slot on the same server that
hosts the DataNode containing the data, and if not, it looks for an empty slot on a machine in the
same rack.
Reference: 24 Interview Questions & Answers for Hadoop MapReduce developers, How
JobTracker schedules a task?