The incorrect option, or the exception to the valid reasons for denial, is (d) the applicant was not a US Citizen.
The valid reasons for denying an applicant's real estate license application are (a) the applicant was previously licensed by the Department and had prior disciplinary history, (b) the applicant forgot to answer all of the questions on the application, and (c) the applicant did not submit fingerprints with the application.
This means that being a US Citizen is not a valid reason for denying the applicant's real estate license application.
It's important to note that citizenship requirements may vary depending on the specific regulations and laws in different jurisdictions. However, in this particular case, the question states that being a US Citizen is not a valid reason for denial.