Today, there are more devices with varying screen sizes and resolutions on the market than ever before. With the majority of these being a mobile device, by using a responsive web design, your site will be viewed appropriately on any device.
Google recommends businesses to go with the responsive design approach because the advantages of having one URL make it easier for users to interact with the page. Using a responsive design also enables Google's bots to index each URL without having to crawl to multiple pages.
Pierre from Google explains in this article:
"As for the separate mobile site configuration, since there are many ways to do this, our recommendation introduces annotations that communicate to our algorithms that your desktop and mobile pages are equivalent in purpose; that is, the new annotations describe the relationship between the desktop and mobile content as alternatives of each other and should be treated as a single entity with each alternative targeting a specific class of device. These annotations will help us discover your smartphone-optimized content and help our algorithms understand the structure of your content, giving it the best chance of performing well in our search results."