Explanation
The best OAuth flow for this scenario is the web server flow. The web server flow is an OAuth authorization flow that allows a web application, such as UC's employee portal, to obtain an access token and a refresh token from Salesforce after the user grants permission. The web application can then use the access token to access Salesforce data and features, such as posting ideas, and use the refresh token to obtain a new access token when the previous one expires or becomes invalid. This flow is suitable for UC's scenario because it allows users to be redirected to Salesforce, authenticated, and presented with the relevant pages when they click on some of the links in the employee portal. This flow also provides a secure and seamless user experience by using a confidential client secret that is stored on the web server and not exposed to the browser.
The other options are not valid OAuth flows for this scenario. The web application flow is not a standard term for OAuth, but it could refer to the user-agent flow, which is an OAuth authorization flow that allows a browser or web-view, such as a mobile app or a desktop app, to obtain an access token from Salesforce by using a script or a pop-up window. This flow is not suitable for UC's scenario, as it does not use a web server or a client secret, and it does not provide a refresh token. The SAML bearer assertion flow is an OAuth authorization flow that allows an external application to obtain an access token from Salesforce by using a SAML assertion from an identity provider (IdP) that verifies the user's identity. This flow is not suitable for UC's scenario, as it does not involve user interaction or redirection to Salesforce. The user-agent flow is an OAuth authorization flow that allows a browser or web-view, such as a mobile app or a desktop app, to obtain an access token from Salesforce by using a script or a pop-up window. This flow is not suitable for UC's scenario, as it does not use a web server or a client secret, and it does not provide a refresh token. References:
[OAuth Authorization Flows], [OAuth 2.0 Web Server Flow for Web App Integration], [OAuth 2.0 User-Agent Flow for Desktop Apps], [OAuth 2.0 SAML Bearer Assertion Flow for Server-to-Server Integration]