how to use dropdown in which only one options which is selected should not be selected by other

Unfortunately, this isn't possible with the dropdown field.

If you're trying to achieve the said requirement using this form, I can see that you have added a GIFT REGISTRY widget which is a workaround for this requirement. So instead of using a dropdown, you can use that widget to limit the options that users can choose.

Let us know if you need further assistance with this.