Select hides under the table in Bootstrap 4

I am beginner web developer. I use in my project Bootstrap 4.

Here is my preview: http://serwer1356363.home.pl/_nauka/ – after click "Liczba sztuk" – select I have problem.

Items from select are hidden under the table (you can’t see everything).

Sorry for not pasting my code here, but from within the code you would not see the problem.

How can I repair it?

Please help.