You can use the ListIndex property to determine which item is selected in a list box or combo box.
Setting
The ListIndex property is an integer from 0 to the total number of items in a list box or combo box minus 1. Microsoft Access sets the ListIndex property value when an item is selected in a list box or list box portion of a combo box. The ListIndex property value of the first item in a list is 0, the value of the second item is 1, and so on.
This property is available only by using a macro or Visual Basic. You can read this property only in Form view and Datasheet view. This property is read-only and isn't available in other views.
Remarks
The ListIndex property value is also available by setting the BoundColumn property to 0 for a combo box or list box. If the BoundColumn property is set to 0, the underlying table field to which the combo box or list box is bound will contain the same value as the ListIndex property setting.
List boxes also have a MultiSelect property that allows the user to select multiple items from the control. When multiple selections are made in a list box, you can determine which items are selected by using the Selected property of the control. The Selected property is an array of values from 0 to the ListCount property value minus 1. For each item in the list box the Selected property will be True (–1) if the item is selected and False (0) if it is not selected.
The ItemsSelected collection also provides a way to access data in the selected rows of a list box or combo box.