What is the best way to get by train from the Mont Saint Michel area to Versailles, France?
Get from where you are to Rennes (by train or bus). Local train schedules can be checked at the website of the TER Basse Normandie.
Take a high-speed (TGV) train from Rennes to Paris Montparnasse. Schedules and prices can be found via the SNCF website. Tickets can be booked via the same site. TGV trains have compulsory reservation. The earlier you book and the more flexible you are, the lower will be the price.
Take a local train to Versailles Rive Gauche. Schedules can be found on the Transilien wesbite. Tickets can be bought on the spot at the Montparnasse station.
Trains between Rennes and Paris are quite frequent. There is one train per hour during the day. There are plenty of trains between Paris and Versailles. The trip to Rennes is probably the bottleneck.
N.B. You may find the following post useful for your purposes: