App Inventor does not have any drop-down menu options currently.
With Blocks
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you can simulate a drop down MENU with a ListPicker How to make a hamburger menu
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you can use a Group Box made from a Slider, or CheckBoxes or a ListView as described here to make a choice; User Feedback on CheckBoxGroup project in GSoC'21 App Inventor
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use an extension How to make a fully customizable side menu
Others may have other ideas.