An app builder has created a custom Lightning App and wants to make it available...

The app builder needs to do two steps to make the custom Lightning App available to the internal users at Universal Containers:

Create a subdomain using My Domain: My Domain is a feature that allows an organization to set up a custom domain name for their Salesforce instance. This is required to use custom Lightning components and apps in Salesforce1.

Build a Custom Tab for the app: A custom tab is a tab that displays any custom object or web page in Salesforce. This is required to make the custom Lightning App accessible from the navigation menu or app launcher2. Option A and B are not necessary steps to make the custom Lightning App available to the internal users.