How can I restrict user to select only one option from a drop down.

https://www.jotform.com/build/81721045627454#preview

In this for I want users to restrict through any available option.

Please confirm urgently about this function.

The form you provided does not have a dropdown field. By default, only one option can be selected from a dropdonw field. 

Quick Overview of Form Fields

Demo - https://form.jotform.com/81721544151955 

Multiple options can be selected in a dropdown field only if Multiple Selections is set to ON inside of the Advanced tab of a dropdwon field.