JotForm is a free online form builder which helps you create online forms without writing a single line of code. No sign-up required.
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Click on Preferences button.
On General Settings, set Responsive Form option to Yes
You can also use our new Form Designer tool if you want to add more styles to the form, and make your form responsive from there.
While on Edit mode, click the Form Designer Icon on the Form Style toolbar. (Pictured below)
On Form Layout click on Make This Form Responsive
Otherwise, by setting the option in Preferences will be enough.