Meaning of "education level"

It's the level you have actually attained.

Don't put down a degree if you don't have one. If there's space for free text, you could add "Current undergraduate student" as that does indicate study at a higher level than high school and at least the possibility of a degree.

The vast majority of employers want to know how much education you have completed. If an employer only wants to know highest degree you have attained, they generally ask just that:

What is the highest degree you have completed/obtained?

Otherwise, they usually want you to tell them exactly how far you have gone. A standard answer to "level of education" would be

Three years toward BA degree.

or

90 credits out of 120 toward BS.

In the previous examples, the completion of a high school program is understood.

There is a significant difference in many employers' eyes between someone who has finished high school and someone who has done most of the course work toward a college or other post-secondary degree.

It is not unusal for graduate students to list the highest level of education as

PhD program in psychology, ABD [all but dissertation]

If there is a specific requirement for a given degree or diploma to be eligible to apply, that will usually be an explicit question asking for date of completion. That doesn't mean that the empoloyer is not still interested in work you may have done toward another educational plateau.