The train is leaving or The train will be leaving.

Both are possible.

I don't think B is incorrect. It really depends on the context which one to use.
If you're at the platform, waiting for your friend to join you, and you can hear the train's engine already running, then A is more suitable because it indicates more certainty.
However, if you're a travel agent, for example, talking to your client on the phone and urging them to hurry to the station so as not

Anonymous I am an EFL student. There's a question on my school test that has been bothering me for a long time.Here's the question:1. The train ___ in five minutes. Please hurry.(A) is leaving (B) will be leavingThe correct answer is (A). But I was wondering why (B) is incorrect.Thanks for your help.

There are different ways to express future actions. S

Or see #10 'Ways of expressing the future' here: https://gramorak.wordpress.com/

Thank you so much for the reply.
It's really hard to tell the difference between these two sentences.
May I ask: which sounds more natural to you?
Thank you:)

Thank you:)
It's really useful.

Thank you for the reply.
I checked grammar book before I asked. But I don't think it responds to my problem well.
Besides, I really want to know which option a native speaker will use in this sentence.

Anonymous I really want to know which option a native speaker will use in this sentence.

I've told you - both are possible. Teechr has explained one possible shade of difference.

The one we native speakers choose depends ultimately on what is going through our subconscious minds at the instant of speaking.