For the selections to be independent events, you must return the marble to the bag before making a selection again. This way, there will be the same number of options each time you make a selection.
For the selections to be dependent events, you must not return the marble. This way each time you make a selection, the conditions for the next one change, making them dependent.
Two events are independent if the result of the second event is not affected by the result of the first event.
On the other hand, two events are dependent if the result of the second event is affected by the result of the first event.
For example in this case, if you return the marble to the bag, the bag will have exactly the same number of marble as before the first selction. It means, the first selection does not affect the second one.
If you don't return the marble, there will not be the same number of marbles in the bag, which completely changes the conditions and affects the result of the second selection.