Final answer:
The subject tickler file is a type of tickler file that helps organize tasks or events based on different subjects or topics.
Step-by-step explanation:
The correct answer is d. Subject.
A tickler file is a system used to remind someone of tasks or events that need to be completed in the future. It consists of a set of folders or containers, each labeled with a different subject or category. The subject tickler file is organized based on the specific subjects or topics related to the tasks or events. For example, if you have tasks related to different subjects like Math, History, English, or Science, you can create separate folders for each subject and file the relevant materials accordingly.