In geology, the geologic time scale is hierarchically subdivided into which of the following major units?

Introduction / Context:
The geologic time scale is a fundamental tool used in earth science to describe the timing and relationships of events in Earth history. It divides the planet's 4.6 billion year history into a nested hierarchy of named intervals. This question tests whether you know the main levels into which geologic time is subdivided and can identify that more than one of the listed units is correct.

Given Data / Assumptions:

- The question asks about the major units used to subdivide the geologic time scale.

- The options include "Eons", "Eras", and "Periods", plus an "All of the above" choice and a distractor about epochs only.

- It is assumed you have basic school level knowledge of earth science and the structure of the time scale.

- The question focuses on broad categories, not on specific names like Precambrian or Mesozoic.

Concept / Approach:
Earth scientists organise the history of the planet into a hierarchy of time intervals. At the broadest level are eons, which are subdivided into eras. Each era is further divided into periods, and many periods are divided into epochs and ages. The correct approach is to recall that eons, eras, and periods are all standard levels in this hierarchy. When a multiple choice question offers several correct categories along with an "All of the above" option, you must check whether each individual category is indeed valid. If they all are, then the answer is "All of the above".

Step-by-Step Solution:
Step 1: Recall that an eon is the largest commonly used division of geologic time, such as the Phanerozoic eon. Step 2: Remember that eons are subdivided into eras, for example the Paleozoic, Mesozoic, and Cenozoic eras within the Phanerozoic eon. Step 3: Note that each era is subdivided into periods, such as the Jurassic or Cretaceous periods. Step 4: Recognise that the time scale indeed uses all three levels: eons, eras, and periods, not just one of them. Step 5: Evaluate the option "Only epochs" and see that epochs are smaller units that do exist but are not the only or main level of division mentioned here. Step 6: Since eons, eras, and periods are all valid subdivisions, choose "All of the above".

Verification / Alternative Check:
You can quickly verify this structure by thinking of a common example from textbooks. Dinosaurs lived during the Mesozoic era within the Phanerozoic eon. Within the Mesozoic era, their history spans several periods, including the Triassic, Jurassic, and Cretaceous periods. Modern humans belong to the Quaternary period within the Cenozoic era of the Phanerozoic eon. This example shows clearly how eons, eras, and periods work together as nested levels of the geologic time scale.

Why Other Options Are Wrong:
Selecting only eons ignores the important subdivisions into eras and periods that make the time scale more detailed and useful. Selecting only eras leaves out the broader eon framework and the finer period subdivisions, both of which are standard in geology. Selecting only periods misses both the larger eon and era levels that set the broader context. Choosing "Only epochs" is incorrect because epochs are actually smaller units nested within periods, and they do not replace eons, eras, or periods as the main subdivisions referred to here.

Common Pitfalls:
A common misconception is to confuse the order of these time units or to believe that one level is more important than the others. Students sometimes mix up periods and epochs or think of them as interchangeable. Remember the hierarchy in order from large to small: eon, era, period, epoch, and age. Keeping this order in mind will help you correctly answer many similar questions on geologic time.

Final Answer:
The geologic time scale is subdivided into All of the above, meaning it uses eons, eras, and periods as major units.