Explanation
The optimal Salesforce license type for all of the UC employees who will access the custom Innovation platform without logging in with Salesforce credentials is the Salesforce Platform license. The Salesforce Platform license allows users to access custom applications built on the Lightning Platform, such as Apex and Visualforce, and use standard objects such as accounts, contacts, reports, dashboards, and custom tabs. It also supports SSO with a third-party identity provider using SAML. Option A is not a good choice because the Identity license is designed for users who need to access Salesforce Identity features, such as identity provider, social sign-on, and user provisioning, but not for users who need to access custom applications. Option B is not a good choice because the Salesforce license is designed for users who need full access to standard CRM and Lightning Platform features, such as leads, opportunities, campaigns, forecasts, and contracts, but it may be unnecessary or expensive for users who only need to access custom applications. Option C is not a good choice because the External Identity license is designed for users who are external to the organization, such as customers or partners, but not for users who are internal employees.
References: Salesforce Help: User License Types, [Salesforce Help: Single Sign-On for Desktop and Mobile Applications using SAML and OAuth]