Regarding Amazon SWF, at times you might want to record information in the workflow history of a workflow execution
that is specific to your use case. ____ enable you to record information in the workflow execution history that you can
use for any custom or scenario-specific purpose.

A.
Markers

B.
Tags

C.
Hash keys

D.
Events

Explanation:
In Amazon SWF, at times you might want to record information in the workflow history of a workflow execution that is
specific to your use case. Markers enable you to record information in the workflow execution history that you can use
for any custom or scenario-specific purpose.
http://docs.aws.amazon.com/amazonswf/latest/developerguide/swf-dg-adv.html