BNBs are bonds issued by NABARD. They are basically zero coupon bonds, meaning you don't get any interest on them.
Assured return schemes tend to account for a significant share of an investor's portfolio. As the name suggests, these schemes offer assured returns i.e. investors know upfront how much return they are going to earn from their investments. Therefore, there is less uncertainty i.e. lower risk associated with investing in such schemes. Notwithstanding the benefits offered by these schemes, not all assured return schemes make worthy investments.