What is the grade failed most often?

There's no single definitive answer to what grade is failed most often. It varies significantly depending on:

* The country/region: Grading systems differ globally.

* The educational level: Failure rates are different in elementary school, high school, and college.

* The subject: Some subjects are inherently more challenging and have higher failure rates than others.

* The institution: Some schools/colleges have stricter grading policies than others.

While anecdotal evidence might suggest a particular grade (like a "D" or "F" in US systems), there's no reliable global statistic to support a single answer.