My parents are currently in US on GC status.They are leavig to India in 2 weeks and stay there
for about one and half year.They are applying for the reentry permit (I-131).

My questions are:

In the reentry permit application (I-131), what should they fill out under "Class of Admision"?
They got their GC through 'F42' Family based immigration (My Dad's brother sponsored them).

Under the "Purpose of trip", would it be sufficient to say 'Family Obligations' or do I have to attach a
statement or letter? They are going to india to take care of my grand mother and settle some family matters.
How can I prove USCIS that they are going there temporarily and will return. Do I have to submit any proof for
what I say in the statement?
Could you please give advice on writing the statement for 'purpose of trip'? Any sample letter would greatly help.

Since my parents are leaving to India in few days, Can I give my address to send the documents to my place
so that I can mail them to my parents once I get the approval documents?

Would they be able to travel back to US using GC within 6 months after applying for the permit
but before the reentry permit has been approved?

If they have decided to come back to US within six months after applying for reentry permit but before the application
is approved, would they be able to cancel the pending reentry permit application?

1. "Class of Admission":
As far as I know (and I do spend a lot of time checking immigration matters), C of A is nowhere defined exactly (is it first time, last time?), so I recommend they write: F42->LPR (if they treveled as GC holders), otherwise just F42
2. "Family obligations" is fine. In the cover letter they can write more; just a few sentences as you wrote here. Use word "temporarily". If they have any proofs (doctor letter), that would be great..
3. They should put on the form that the re-entry permits should be mailed to the consulate in India. The consulate will notify them when they are approved.. and they can pick them up there.
4. I am 99% sure that there wan't be any problem if they come back within 6 months, with or without having the re-entry permit.

USCIS issues re-entry permits automaticly to almost enyone who applys. Re-entry permit may be denied if person just became PR (and hasn't really yet established a domicile in the USA) or has already spent a lot of time outside the USA.