Okay, so the prescriptive process model describes traditional software processes.
This is very early in the TB.
This is a bit of an insight question-you have to describe why the use of the tradional models bring structure to SD.
I would think something in the line of:
1) They provide a sequence for the generic framework activities.
2) This forms a structure the designer can follow.
3) They prescribe a set of framework activities that can be used for each project, which will formalize the project.
For b, I would think of disadvantages:
1) Real projects rarely follow sequential flow
2) A working product is only delivered late in project
3) Continuous change...
4) Tight time lines...
5) etc.
I used the opposite of the positives for Agile process to help me here, as they are the reason to prefer it above traditional process