Responsive design is a web design approach that aims to make web pages adapt to different screen sizes and resolutions, ensuring good usability and user experience across all devices. Some of the best practices for responsive design are:

Specify breakpoint sizes: Breakpoints are the points at which the layout of a web page changes based on the width of the viewport. For example, a web page may have a two-column layout on a desktop, a single-column layout on a tablet, and a stacked layout on a mobile phone. Specifying breakpoint sizes helps to create a fluid and flexible layout that responds to the device capabilities and user preferences. Breakpoints can be specified using media queries in CSS, which allow applying different styles depending on the media features, such as width, height, orientation, resolution, etc. For example:

@media (max-width: 600px) { /* Styles for screens that are 600px or smaller */ }

@media (min-width: 601px) and (max-width: 900px) { /* Styles for screens that are between 601px and 900px */ }

@media (min-width: 901px) { /* Styles for screens that are 901px or larger */ }

Minimize page weight: Page weight is the amount of data that a web page transfers to load on a browser. It includes the size of the HTML, CSS, JavaScript, images, fonts, and other resources that make up the web page. Minimizing page weight helps to improve the performance, speed, and user satisfaction of a web page, especially on mobile devices that may have limited bandwidth, battery, and processing power. Some of the ways to minimize page weight are:

Optimize images: Images are often the largest contributors to page weight, so it is important to optimize them for the web. This means choosing the right format, size, resolution, and compression level for each image, as well as using responsive images techniques, such as the srcset and sizes attributes, to deliver the most appropriate image for each device and screen size.

Implement caching: Caching is a technique that stores a copy of a web page or its resources on the browser or the server, so that they can be reused without having to be downloaded again. This reduces the amount of data that needs to be transferred and improves the loading time of a web page. Caching can be implemented using HTTP headers, such as Cache-Control and Expires, or using service workers, which are scripts that run in the background and intercept network requests.

Minify and concatenate files: Minification is a process that removes unnecessary characters, such as whitespace, comments, and formatting, from the code files, such as HTML, CSS, and JavaScript, to reduce their size. Concatenation is a process that combines multiple code files into one, to reduce the number of HTTP requests that the browser needs to make. Both minification and concatenation can help to reduce the page weight and improve the performance of a web page.

Use a content delivery network (CDN): A CDN is a network of servers that are distributed across different locations and regions, and that store and deliver copies of a web page or its resources to the users. A CDN can help to reduce the page weight and improve the speed of a web page by serving the content from the nearest server to the user, reducing the latency and bandwidth consumption.

The other two options, using separate URLs per device and utilizing pop-up windows, are not considered responsive design best practices, as they can create usability and accessibility issues for the users. Using separate URLs per device means creating different versions of a web page for different devices, such as example.com for desktop, m.example.com for mobile, and t.example.com for tablet. This approach can lead to inconsistent and fragmented user experiences, as well as duplicate content and SEO problems. Utilizing pop-up windows means creating new browser windows that open on top of the current web page, usually to display advertisements, notifications, or forms. This approach can be annoying and intrusive for the users, as well as difficult to close or navigate on small screens.