** Using WeBWorK **

The MathObject answer checking mechanism has several layers, with different hooks that make customization possible. The OneOf object hooks into the cmp_compare() method, which is a second-level function, and calls the individual cmp_compare() methods of the different choices in the OneOf. Unfortunately, the NumberWithUnits() object hooks into a higher-level function, cmp_parse(), and that is where the unit is located and managed. So the NumberWithUnits object's cmp_parse() method is never calls, and that is why your problem is complaining about the degK.

This is certainly a limitation of the OneOf implementation, if not an outright bug. The OneOf object should probably use its top-level cmp_parse() method to call evaluate() on all the choices individual answer checkers looking for a correct answer among them, rather than taking the shortcut of calling their cmp_compare(). This was done for efficiency, but it is probably more reliable to call cmp->evaluate() instead.

Fortunately, you can do this easily enough yourself for now. You can add

package myOneOf;
our @ISA = ('parser::OneOf');

sub cmp_parse {
  my $self = shift; my $ans = shift;
  my $student = $ans->{original_student_ans};
  my $max = 0;
  foreach $choice (@{$self->{data}}) {
    my $hash = $choice->cmp->evaluate($student);
    if ($hash->{score} >= $max) {
      foreach my $name ('correct_ans', 'correct_ans_latex_string', 'correct_value') {
        $hash->{$name} = $ans->{$name};
      }
      $ans = $hash;
      $max = $hash->{score};
    }
  }
  return $ans;
}

package main;

sub myOneOf {myOneOf->new(@_)}

to the top of the problem, and then use myOneOf() in place of OneOf() and it should work with NumberWithUnits objects as well as other ones.

I will make a pull request to adjust OneOf to work this way, but for now you can use this work-around.