Hello!

I see that you are applying to Mumbai University. In the year of passing, mention the year that you graduated from the school in and its corresponding roll number. But as you have given an improvement exam, if there is a remarks section in the application form, be sure to mention it. Typically, only if your score is higher than your original, it will be considered. Else, it is only the best of the two.

All the best!

If you have given improvement exam but you have passed in your actual boards in class 12th, you have to mention the school you passed your exam from, the actual year of passing and roll no that was provided in the first place.

Because the passing certificate is just issued once when you actually pass your boards. 

If you have scored better in your improvement exam, all you will have to do is present it at the time of admission so that it is counted too, otherwise if you have scored low, you do not even need to present it.