In the Name of God, the Compassionate, the Merciful.
Hi!
A: Hello, I need to register for a class.
B: What class are you trying to take?
A: I want to take a Psychology class.
B: Well, there are only two classes open.
A: Can you tell me what days the classes are on?
B: One class is on Tuesday and Thursday from 2:00 p.m. - 4:00 p.m.
A: And the other class?
B: That class is from ten to twelve on Monday and Wednesday.
What is the best synonym for “open” here?
Thank you
It means they have room for more students. If a class has no more room it is said to be full.
You can say “It is open for enrollment.” However this can have a second meaning. For example if a class starts on August 20, enrollment might start three months prior to that on May 20. So it might be said "Enrollment opens on May 20.
a good synonym would be: available for enrollment or participation.
This one shifts the focus to timing and tells us when exactly the classes take place.
Thank you so much, NearlyNapping 
Very nice.
To me, the word AVAILABLE seems to be a close synonym.
Thank you, my good friend.
Thank you so much, Anglophile
Very nice.