Hey Risabh,

There is no minimum eligibility to appear in CAT. But some of the IIMs give almost 10% weightage to 12th standard while some may give up to 50%.

But as per the norms, in your case you have appeared NIOS and made your percentage to 80. You can apply with the 80% stat only if you have got a certificate from govt. showing your grade. If you have it, than during verification you will show that particular certificate and ignore the one with the lower grade. You can most certainly do that even if you graduated in a college, with the lower grade.

All the best!