Paris CDG Airport to Montparnasse - Train station

Taxi from the official taxi stand is 58 EUR fixed price to Gare Montparnasse. That would be the most efficient.

Or, you could take the RER B train to Denfert-Rochereau, then the #6 metro to Gare Montparnasse.

As Inbsig writes , take the RER B to Denfert - Rochereau , but instead of the #6 metro where you will be dealing with a fair number of steps ,up and down with your bags , cross the street and jump on either a 59 0r 88 bus . no up and down ,and 5 minutes to Place de Catalogne , across the street from the station . Also covered with the trip from the airport by your Navigo Decouverte card https://www.google.com/maps/dir/Froidevaux+-+Gassendi,+75014+Paris,+France/Pl.+de+Catalogne,+75014+Paris,+France/@48.8368045,2.3214263,16.75z/data=!4m14!4m13!1m5!1m1!1s0x47e671b61e8d8f85:0x70be0fef29493516!2m2!1d2.3282661!2d48.8358155!1m5!1m1!1s0x47e670357ab50501:0xfdd2f4ba83a98b77!2m2!1d2.3186143!2d48.8369826!3e3