What is the difference between a failed task attempt and a killed task attempt?

A.
A failed task attempt is a task attempt that threw an unhandled exception. A killed task attempt
is one that was terminated by the JobTracker.

B.
A failed task attempt is a task attempt that did not generate any key value pairs. A killed task
attempt is a task attempt that threw an exception, and thus killed by the execution framework.

C.
A failed task attempt is a task attempt that completed, but with an unexpected status value. A
killed task attempt is a duplicate copy of a task attempt that was started as part of speculative
execution.

D.
A failed task attempt is a task attempt that threw a RuntimeException (i.e., the task fails). A
killed task attempt is a task attempt that threw any other type of exception (e.g., IOException); the
execution framework catches these exceptions and reports them as killed.

Explanation:
Note:
*Hadoop uses “speculative execution.” The same task may be started on multiple boxes. The first
one to finish wins, and the other copies are killed.
Failed tasks are tasks that error out.
*There are a few reasons Hadoop can kill tasks by his own decisions:
a) Task does not report progress during timeout (default is 10 minutes)
b) FairScheduler or CapacityScheduler needs the slot for some other pool (FairScheduler) or
queue (CapacityScheduler).
c) Speculative execution causes results of task not to be needed since it has completed on other
place.
Reference:Difference failed tasks vs killed tasks