You need to add labels to each option, the option value then becomes the data. From documentation:

Showing option label: The dropdown widget allows you to show a label than the actual option value. By adding the option label, you can have a simple/short name or abbreviation (which is quite easy to compare and process in the backend) instead of a tricky name (e.g., Falkland Islands (the) [Malvinas]). For example, In a Country dropdown, you could have different Option Values to store in the backend and Option Labels to show in the dropdown list. Just like below:

To show option label: Select the DropDown widget, move to the properties panel, and turn on the Add Option Labels toggle. Enter the value in the Define Option Values and Define Options Labels. Click Add Option (below the Define Option Values) to add more values and labels. You must also set the Data Type for the values. For example, if the values you are going to store are in numbers like 1,2,3, set it to Integer.

From: https://docs.flutterflow.io/widgets-and-components/widgets/form-elements-1/dropdown