Universal containers (UC) uses a legacy Employee portal for their employeesto collaborate and post their...

Adding the employee portal’s IP address to the trusted IP range for the connected app is the best way to restrict the connection to Salesforce only to the employee portal server. This will ensure that only requests from the specified IP range will be accepted by Salesforce for that connected app. Option B is not a good choice because using a digital certificate signed by the employee portal server may not be supported by Salesforce for OAuth username-password flow. Option C is not a good choice because adding the employee portal’s IP address to the login IP range on the user profile may not be sufficient, as it will still allow other users with the same profile to log in from that IP range. Option D is not a good choice because using a dedicated profile for the user that the employee portal uses may not be effective, as it will still allow other users with that profile to log in from any IP address. References: [Connected Apps], [OAuth 2.0 Username-Password Flow]