How can the connection to salesforce be restricted only to the employee portal server?

Universal containers (UC) uses a legacy Employee portal for their employees to collaborate and post their ideas. UC decides to use salesforce ideas for voting and better tracking purposes. To avoid provisioning users on Salesforce, UC decides to push ideas posted on the Employee portal to salesforce through API. UC decides to use an API user using Oauth Username – password flow for the connection .

How can the connection to salesforce be restricted only to the employee portal server?
A . Add the Employee portals IP address to the Trusted IP range for the connected App
B . Use a digital certificate signed by the employee portal Server.
C . Add the employee portals IP address to the login IP range on the user profile.
D . Use a dedicated profile for the user the Employee portal uses.

Answer: A