What ethnicity to mark?

You can select "Other ethnicity" or "Prefer not to say." Note that, according to ColllegeVine, picking Asian greatly reduces your chances at competitive schools.

You can select "Other ethnicity, biracial, or multiracial" in the chancing profile!

This can be a pretty difficult question for a lot of people. I did find this article which helped a bit https://blog.collegevine.com/faq-about-the-race-ethnicity-section-of-the-common-application/. It looks like it links to other articles too which could be helpful. I'm not sure about other applications but I know at least for the common app you can skip the entire demographics information section if you want. You can also pick multiple races/ethnicities on the common app I believe.

To answer your question though, what do you identify as more? I would pick whatever you identify more as since we can only pick one ethnicity on CollegeVine for now. And just so you know, just because you pass as only white doesn't mean you can't put down Asian on the application if that's what you want to do.