Option C would be correct. By mentioning the reason for the gap in the Covering Letter.
A covering letter is a document, attached with the CV or Resumes, that describes the content of the attached documents. If anything needs to be mentioned by the Resume Holder, that should be there in the cover letter.
There is no need to write it down in the beginning or the ending of the Resume, or there is no need to mention the reason in the portfolio. Covering Letter is the best way to write about such problems.