In order to get  list of Report grades where a student has a D or below we can do the following:

1. Go to Academics > Grades > Report Card Grades List
2. Select filters 
3. Expand Grades.
4. Under include select numeric or letter grades(depends on what style of grading you want to look at)
5. In the next box change show all to Less than or equal to
6. In the last box enter the value of D or 70 for example
7. Now select grade plan grades
8. Select the grading period you are looking to review(ex. First semester,Q1, Tri1 etc.)
9  Apply filters

After you apply the filters you will have a list of report card grades that are D or below.