Why grades from 0% to 100% considered discrete? It can be 67.95% which means it's not an integer.

The percentage is discrete or continuous based on the data whose percentage is being calculated. If the percentage of discrete is being calculated then, the percentage will be discrete, and if the percentage of continuous is being calculated then continuous. In above question, since the marks given to any student will be discrete and total marks will also be discrete. So, the percentage will be discrete.

Maybe the context is the multiple choices questions with only one answer

discreate here as my understanding for the grade it is a specific value that doesn't change with time so it can be plotted by a single point on the graph