What is the optimal Salesforce licence type for all of the UC employees?

Universal Containers (UC) is building a custom Innovation platform on their Salesforce instance. The Innovation platform will be written completely in Apex and Visualforce and will use custom objects to store the Data. UC would like all users to be able to access the system without having to log in with Salesforce credentials. UC will utilize a third-party idp using SAML SSO.

What is the optimal Salesforce licence type for all of the UC employees?
A . Identity Licence.
B. Salesforce Licence.
C. External Identity Licence.
D. Salesforce Platform Licence.

Answer: D