All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
We have already clarified and defined Mudhaarabah in the Fatwa to which you referred. Therefore, you should compare this to the reality of National Bonds which you inquired about.
There are two important things to mention here:
1- Guaranteeing capital money: If the capital money in the National Bonds is guaranteed, then the transaction is nothing but a loan with Riba (interest and/or usury) and it is not a permissible Mudhaarabah. In this case it is not permissible for you to take part in it.
2- The field of investment should be permissible, and it should not be in what Allaah has forbidden, like intoxicants, dead meat, building places of dissoluteness and the like.
Hence, you should not be so worried and confused; by asking the people in charge of this National Bonds about its field of work and guaranteeing the money, the ruling will become clear and the problem solved.