Ans:- First of all you need to make a Affidavit(affidavit can be made from block office or court near by you meeting a layer(additionally if you dont know about affidavit you can search in google or you may take help from your parents)) and second thing you need is a medical report medical report must be of a long illness of about six month and more(you can make a medical report from your family doctor or any doctor who may help you regarding this) medical report should be dated in between your 10th pass and 11th admissions date for 10th pass date you can see your 10th result there you will find the date along with month and year and for Date of admission in 11th you can see your College Leaving Certificate or Tranfer certificate ther you can find date of admission.
answer will be very simple say that I was not physically fit thats why I have not taken admission that year as I was suffering from jondis (replace jondis with your illness statement from your medical report) am having affidavit and medical documents along with me to suuport this.
if they additonally ask you which is very rare that why don’t you taken admission and given the final exam (11th) without attending the classes than simply answer if i have done this i will have no concept of 11th which is mandatory for 12th Board exams.
ques:-why affidavit and medical report is needed?
ans:-Affidavit is needed just to verify that you was not involved in any criminal activity and to support your ans both are really very helpfull.