How to convert multiple two option set values into one option set?

Hello Experts,

I have 1:N relationship between Account and a custom entity called Students.

Account Entity has three " Two Option Set" {check boxes} fields: A, B , C. The user can only select one and only one field out of these fields.   We have a student grid on the accounts form. 

The requirement is on creation of a student record a new field "Class" which is an option set should be auto populated with the field selected in account.

For example:  Users selects value of field A in Account entity as yes, then on creation of student record Class should be auto populated with "A".

Thanks in advance.