Hello my Fellow!
Being responsive is the quality of the theme or template you are using. So you should ask for a theme that would be having "responsiveness". It's always recommended to ask the seller about the receptive feature of the theme/template, otherwise you can make a theme/template yourself using a responsive generator like TemplateToaster.
And also,
I want you to let know that all of the platforms you have asked to suggest you best from, are good. It depends on your technical expertise that you can make best use of which platform. If you want a very simple platform to work with then i would recommend you WordPress. Joomla and magento are a bit more technical if have to make their best use.