Explanation
The web server flow is used for web applications when server-side code needs to interact with APIs. This flow implements the OAuth 2.0 authorization code grant type, which allows the web app to obtain an access token and a refresh token from Salesforce after the user grants permission1. The web app can then use the access token to call the Salesforce APIs and use the refresh token to obtain a new access token when the previous one expires2. The other options are not valid scenarios for using the web server flow. The web server flow is not used for server-side components when page needs to be rendered, as this does not involve API calls. The web server flow is not used for mobile applications and testing legacy integrations, as these scenarios are better suited for other OAuth flows, such as the user-agent flow or the password flow3. The web server flow is not used for verifying access protected resources, as this is a general purpose of OAuth, not a specific scenario for the web server flow. References: OAuth 2.0 Web Server Flow for Web App Integration, Mastering Salesforce Canvas Apps, OAuth Authorization Flows