Answer:
<h2>By the common knowledge and similar skill sets required</h2>
Explanation:
The most reasonable answer is the second option.
An occupation refers to a profession, a job.
An occupation is grouped according to the knowledge required and all skill set needed to do that specific job. This way, people can also find other occupations related with their field.
On the other hand, it doesn't make sense that occupations are grouped by age, names, or levels of education, it's not like that. It's true that an occupation could have different level of specialization, which are determined by the level of education, however, that's only about the hierarchy of those occupations.
Therefore, the right answer is the second choice.
