Can A PPL Holder File A Flight Plan?

[Easy226]

Hi everyone,just a few questions i thought i would ask:

1) Can a PPL holder file a flight plan to fly in controlled airspace (i.e. class D) if the weather is classed as VMC

2) How and when do you file a flight plan?? Also, how are they constructed, i.e. is there a procedure for constructing one??

3) From my Air Law manual it states as an example:
D129/1.75 indicates between 51 degrees north and 52 degrees north from sfc-1,750ft AMSL. I understand the altitude part, but i dont understand how Danger area 129 can represent an area between 51 and 52 degrees north??

Thanks,
DAN