I have done a mistakes at the time of form filling.. when it's time to select the subject.!
first of all check whether the coorections can be made or not . the bsc is done with one subject as the main subject so if it is not a problem to chose your prefered subject then don't be panic.however if nothing can be done then reach before or on the day of counselling and contact tge college administration whether you can change you subject or not . otherwise participate in the counselling and ask for your preferred subject