Train a arrives at the train station every 30 minutes, and train b arrives every 40 minutes. both are at the train station right now. in how many minutes will both be at the train station again?

You're looking for the smallest number that's a multiple of both 30 and 40.
If you've learned about "Least Common Multiple" by now, you'll be happy
to know that that's exactly what this is, (because when you were learning it,
you wondered what in the world you could ever use it for).  Well here you are.

The least common multiple of 30 and 40 is 120.
That's the 4th multiple of 30 and the 3rd multiple of 40.

If the trains are both in the station right now, then the next time they'll
both be here together will be 2 hours (120 minutes) from now.

And let me say in conclusion that when you ask for help with
a multiple-choice question, you must also post the choices that
you have.  It's not fair to the helper if he doesn't know the choices.