In CDG airport (Paris) there are RER ticket vendor machine. There are few options on the screen as follows:
(1) ticket for within paris city,
(2) ticket for paris region, etc
I will go from Montparnasse gare from CDG airport.
Which option should I choose ?
Can I use my International Master card (Debit)?
Where to exchange money at the airport ?
Gare Montparnasse is inside Paris, so "ticket to Paris" should be the right option. The exact wording varies between machines and locations, but it is probably the first available option on the screen on machines at CDG RER station. There's usually staff standing by to help you if you have any doubt.
Do not take "ticket t+". Those are valid only for metro (everywhere), RER (only inside Paris city limits, known as Zone 1), bus (everywhere) and tram (everywhere).
In the worst case, you would have an option for "tickets for the Paris region" which then asks you where you are going.
As already stated in an answer or comment to one of your previous questions, you can indeed use a Mastercard debit card, provided it is chip & pin, you known the pin, and your bank doesn't block the transaction (which they may do for foreign transactions if you don't warn them beforehand).
You can definitely exchange money at the airport, but the best option (in terms of exchange rate/fees) is usually to withdraw money at an ATM with you Mastercard debit card. Same caveats apply (know the pin, make sure your bank is aware before you leave). Check you card's fees beforehand, though.
