Delegated Authentication is an authentication mechanism that allows Salesforce to delegate the authentication process to an external system via a SOAP webservice. The external system can manage the user administration, passwords, and authentication requests.The other options are either not suitable or not supported for this use case. References: Delegated Authentication, FAQs for Delegated Authentication

Single sign-on (SSO) is an authentication method that enables users to access multiple applications with one login and one set of credentials4. There are two types of SSO flows thatcan be used with Salesforce as the service provider (SP) and an external identity provider(IdP)5:
* Service-provider-initiated SSO: The user requests a resource from the SP, such as a Salesforce URL.
The SP redirects the user to the IdP for authentication. The IdP authenticates the user and sends a SAML response to the SP. The SP validates the SAML response and grants access to the user5. This type of SSO flowsupports deep linking, which means that the user can access a specific page within Salesforce without logging in again6.
* Identity-provider-initiated SSO: The user logs in to the IdP and selects an app from a list of available apps. The IdP sends a SAML response to the SP. The SP validates the SAML response and grants access to the user5. This type of SSO flow does not support deep linking, which means that the user can only access the default landing page of Salesforce6.
References:
Single Sign-On
SAMLSSO Flows
Deep Linking

JWT OAuth Flow is a protocol that allows a client app to obtain an access token from Salesforce by using a JSON Web Token (JWT)instead of an authorization code. The JWT contains information about the client app and the user who wants to access Salesforce. To use this flow, the client app needs to have a connected app configured in Salesforce. The connected app is a framework thatenables an external application to integrate with Salesforce using APIs and standard protocols. To support JWT OAuth Flow, two settings need to be configured in the connected app:
* The Use Digital Signature option, which enables the connected app to verifythe signature of the JWT using a certificate.
* The "api" OAuth scope, which allows the connected app to access Salesforce APIs on behalf of the user. References: JWT OAuth Flow, Connected Apps, OAuth Scopes

IDP-initiated SSO flow is when the user starts at the identity provider (IDP) site and then is redirected to the service provider (SP) site with a SAMLassertion. This flow is suitable for UC's scenario because they want to use their existing portal as the primary site and also enable seamless access to the partner community. The IDP-initiated flow does not require the user to log in again at the SP site,which is Salesforce in this case.
References: SAML SSO Flows, Single Sign-On, Salesforce Community Single Sign-on (SSO)

To support scalability and reduce maintenance costs for a multinational, multi-brand organization, the architect should recommend assigning each sub-brand a unique Experience ID and using the Experience ID to dynamically brand the login experience. Experience ID is a parameter that can be used to identify different brands or experiences within a single Experience Cloud site (formerly known as Community). Dynamic branding is afeature that allows Experience Cloud sites to display different branding elements, such as logos, colors, or images, based on the Experience ID or other criteria. This solution can provide a consistent and personalized brand experience for each sub-brandwithout creating multiple subdomains or orgs. References:
Experience ID, Dynamic Branding for Experience Cloud Sites