OK, You should be able to find all ofthis in your books as they are fairly basic...
1. One of the functions of a hydraulic accumulator is..to asborb power surges from the pump, alo in some systems to provide emergency actuation in the event of pressure loss eg. brakes
2. The answer would seem to be "c" but most aircraft pressure controllers will be trying to acheive a negative diff px of .125 psi. (boeing yes, not sure about the airbus)
3. When "start" is selected and the APU is running and fully online. The packs will either turn off automatically or have to be seleced off by the crew as part of the start checklist. Electrical supply will remain unaffected until Engine Gene's come up to speed