If this is a grammar police post you're wrong; an nc is correct, an national championship would be wrong.
Since UNL already has five(?) national championships that is the correct answer. Iowa's "disputed" championship is a joke as something like nineteen of twenty polls at the time gave it to OU that year.
If the question is who will win one from this point forward I'd say none of the above although Nebraska's chances will increase significantly by joining the B10.