Explanation
The Salesforce Community Cloud should support OpenId Connect Authentication Provider and Registration Handler, and SAML SSO and just-in-time provisioning for self-registration and SSO. OpenId Connect is a protocol that allows users to authenticate with an external identity provider and access Salesforce resources. A registration handler is a class that implements the Auth.RegistrationHandler interface and defines how to create new users or update existing users in Salesforce from the information received from the identity provider. SAML SSO is a protocol that allows users to log in to Salesforce with a single credential from an identity provider. Just-in-time provisioning is a feature that allows creating or updating user accounts in Salesforce based on SAML assertions.
References: [OpenID Connect Authentication Providers], [Registration Handler Interface], [SAML Single Sign-On for Communities].

Explanation
The integration architect should recommend using Salesforce Connect to display the financial transactions as an external object. Salesforce Connect is a feature that allows you to integrate external data sources with Salesforce and access them in real time via external objects. External objects are similar to custom objects, but they store metadata only and not data. You can use external objects to display data from the core banking system in the customer community without copying or syncing the data. You can also use standard Salesforce features, such as reports, dashboards, or global search, with external objects. Salesforce External Service is a feature that allows you to import an external schema definition and generate an Apex wrapper class that can invoke an external service. This is useful for integrating complex business processes or workflows with Salesforce, but not for displaying data in a community lightning page. Iframe is a HTML element that allows you to embed another web page within a web page. This is not a recommended solution for displaying data in a customer community, as it can pose security risks, performance issues, or user interface problems.
References: [Salesforce Connect], [Salesforce External Services], [Using Iframes in Lightning Components]

Explanation
The first thing an integration architect should validate if a callout from a Lightning Web Component to an external endpoint is failing is the endpoint domain has been added to Cross-Origin Resource Sharing (CORS).
CORS is a mechanism that allows web browsers to make requests to servers on different origins, such as different domains, protocols, or ports. CORS requires the server to send back a special header that indicates whether the browser is allowed to access the resource or not. If the endpoint domain is not added to the CORS whitelist in Salesforce, the browser will block the callout and throw an error. Option B is not correct because Content Security Policies (CSP) are used to control what resources can be loaded on a Visualforce or Lightning page, such as scripts, stylesheets, images, etc. CSP does not affect the callout from a Lightning Web Component to an external endpoint. Option C is not correct because outbound firewall rules are used to restrict the network traffic from Salesforce to external systems. Firewall rules are configured at the network level, not at the Salesforce level. Option D is not correct because Remote Site Settings are used to specify the domains that are allowed for callouts from Apex code, not from Lightning Web Components. References:
Working with CORS and CSP to Call APIs from LWC
[Cross-Origin Resource Sharing (CORS)]