Hi Aisha! What caste you fill in probably does not affect your rank, caste will come into play when you appply for some counselling and in that case the respective cutoff of your category (or caste) will be considered. Putting it the other way so that it becomes clearer to you; caste based cutoff marks and rank are two separate things, the rank which is calculated is same for all the General/OBC/SC/ST, it is only the cutoff that varies. But since it is mandatory to provide authentic information you must contact the concerned authorities and share with them about this issue and request them a change in the 'caste' option, simply write to them on e-mail id provided on their website and also give them a call. All the contact details will be available on the examination controller's website.