Explanation
The SAML SSO flow that an architect should recommend for UC is IdP-initiated. IdP-initiated SSO is a process that allows users to start at the IdP site, such as UC's custom web page, and then be redirected to Salesforce or other SPs with a SAML assertion that contains information about the user's identity and attributes. This flow enables UC to provide a single point of entry for its users to access multiple applications with the same credentials, as they do not need to enter their username and password again for each application.
This flow also simplifies the configuration and maintenance of SSO, as UC does not need to create or manage deep links or URLs for each application.
The other options are not valid SAML SSO flows for this scenario. SP-initiated with deep linking is a process that allows users to start at a specific resource on the SP site, such as a report or dashboard, and then be redirected to the IdP for authentication and back to the resource with a SAML assertion. This flow is not suitable for UC's scenario, as they want their users to start at their custom web page, not at a specific resource on Salesforce or other SPs. SP-initiated is a process that allows users to start at the SP site, such as Salesforce or other applications, and then be redirected to the IdP for authentication and back to the SP site with a SAML assertion. This flow is not suitable for UC's scenario, as they want their users to start at their custom web page, not at each application separately. User-agent is not a standard term for SAML SSO, but it could refer to user-agent flow, which is an OAuth authorization flow that allows users to obtain an access token from Salesforce by using a browser or web-view. This flow is not suitable for UC's scenario, as it does not use SAML or IdP for authentication. References: [SAML Single Sign-On], [IdP-Initiated Login], [SP-Initiated Login], [Deep Linking], [OAuth User-Agent Flow]