Explanation
The underlying mechanisms that the UC Architect must ensure are part of the product are Just-in-Time (JIT) provisioning and deprovisioning. JIT provisioning is a process that creates or updates user accounts in Salesforce when users log in with SAML single sign-on (SSO)6. JIT deprovisioning is a process that disables or deletes user accounts in Salesforce when users are removed from the identity provider (IdP). Both of these processes enable automated provisioning and deprovisioning of users without requiring manual intervention or synchronization. The other options are not valid mechanisms for provisioning and deprovisioning. SOAP API is an application programming interface that allows developers to create, retrieve, update, or delete records in Salesforce. However, SOAP API does not support JIT provisioning or deprovisioning, and requires custom code to implement. Provisioning API is not a standard term for Salesforce, and there is no such API that supports both provisioning and deprovisioning.
References: Just-in-Time Provisioning for SAML, [Just-in-Time Deprovisioning], [SOAP API Developer Guide]

Explanation
A SAML SSO 'Replay Detected and Assertion Invalid' error occurs when Salesforce detects that the same assertion has been used more than once within the validity period. This can happen if the assertion ID is reused by the IdP or if the assertion is resent by the user. Another possible cause is that the time settings of the IdP and Salesforce are not synchronized, which can result in an assertion being valid for a shorter or longer period than expected. References: SAML Single Sign-On Settings, Troubleshoot SAML Single Sign-On

Explanation
Just-in-time provisioning is a feature that allows Salesforce to create user accounts automatically when users log in for the first time via an external identity provider. This way, UC can avoid creating user records manually or synchronizing them with another system. On-the-fly provisioning is not a valid term in Salesforce.
Salesforce APIs can be used to create users programmatically, but they are not related to SSO. Identity Connect is a tool that can sync users between Salesforce and Active Directory, but it is not required for SSO.
References: Certification - Identity and Access Management Architect - Trailhead, [Just-in-Time Provisioning for SAML and OpenID Connect]

Explanation
A is correct because creating a connected app that supports the JWT Bearer Token OAuth Flow allows the middleware to authenticate to Salesforce without storing usernames and passwords. The JWT Bearer Token OAuth Flow uses a certificate and a private key to sign a JSON Web Token (JWT) that contains information about the user identity and requested access. The middleware sends the JWT to Salesforce, which verifies it using the certificate and grants an access token2.
B is incorrect because creating a connected app that supports the Refresh Token OAuth Flow requires storing usernames and passwords in the middleware. The Refresh Token OAuth Flow uses a username-password authentication flow to obtain an access token and a refresh token. The middleware can use the refresh token to obtain new access tokens without user interaction, but it still needs to store the username and password for the initial authentication3.
C is incorrect because creating a connected app that supports the Web Server OAuth Flow requires user interaction to authenticate to Salesforce. The Web Server OAuth Flow redirects the user to a Salesforce login page, where they enter their credentials and grant access to the middleware. The middleware then receives an authorization code that it can exchange for an access token and a refresh token4.
D is incorrect because creating a connected app that supports the User-Agent OAuth Flow also requires user interaction to authenticate to Salesforce. The User-Agent OAuth Flow is similar to the Web Server OAuth Flow, except that it does not return a refresh token. The middleware can only use the access token until it expires5.
References: 2: Accessing Salesforce with JWT OAuth Flow 3: OAuth Authorization Flows - Salesforce 4: OAuth Authorization Flows - Salesforce 5: OAuth Authorization Flows - Salesforce

Explanation
The two role combinations that are represented by the systems in the scenario are Salesforce as the service provider and Google as the identity provider, and Salesforce as the service provider and Facebook as the identity provider. This means that Salesforce hosts the customer community app and relies on Google or Facebook to authenticate the users who log in with those options4. Therefore, option B and D are the correct answers.
References: Salesforce as Service Provider and Identity Provider for SSO