Is it possible to get from terminal a to terminal c with with only 40min connection time?

Yes Airtrain But TSA in terminal C is usually packed, taking 40 minutes alone to get through.

You're assuming planes are on time. Yes, it's possible on the Airtrain, but remember that Newark can be very crowded, depending on the time of day. You'll have to go through a TSA check again, too

Yes, just use the AIRTRAIN. It's free and runs every 5 min.