How do these three parameterized variables differ?

Given AGenericClass declared as below:

public class AGenericClass<T> {
  T subject;
  public void setSubject(T subject) {
    this.subject = subject;
  }
}

What are the differences between variables a, b, and c?

AGenericClass<String> a = new AGenericClass<>();
AGenericClass<?> b = new AGenericClass<String>();
AGenericClass c = new AGenericClass<String>();

a.setSubject("L"); // OK.

b.setSubject("M"); // Error: setSubject(capture<?>) cannot be
                   // applied to (java.lang.String)

c.setSubject("N"); // Warning: Unchecked call to 'setSubject(T)'
                   // as a member of raw type 'AGenericClass'

a b and c all are declared without complaint from the IDE, but they all behave differently when setSubject is called.