Question

give reason a screw driver is used to unscrew the rusted nut​

Answer 1

Answer:

Depending on how rusted the bolt head is, it may have become distorted. Either rounded at the corners or swollen. If either of these have happened then a spanner or socket may not fit properly.

One possible option is the cut a slot in the bolt head and use a flat headed screwdriver in the slot to back it out.