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.

When the Allow Customer Invitations setting is enabled, customers (external users) can be invited to specific Chatter groups as external members, allowing collaboration and communication within those groups only.
Why A (The ability to interact with members of their groups) is correct:Exact Extract:"External users invited to Chatter groups can post, comment, and interact with members of the groups they belong to, but can't access information outside those groups." - Salesforce Help | External Users in Chatter Groups Why not B:External users cannot @mention accounts; they can only mention members of their groups.
Why not C:External users cannot invite others to groups - only internal members or group owners can.
Why not D:External users cannot request to join public groups unless invited.
Reference:
Salesforce Help | External Users in Chatter Groups
Salesforce Help | Enable Customer Invitations
Salesforce Platform App Builder Exam Guide | Mobile and Collaboration

If an employee attempts to edit the description field after submitting an opportunity for approval, they will be presented with a 'Record Lock' notification. This means that the record is locked for editing while it is in the approval process and only users with the "Modify All" permission or users above them in the role hierarchy can edit it. [Source]

The best solution to enable reps to see and access the most-used components from anywhere within the app without leaving the page they are working on is Utility bar. This is a footer that can be added to any Lightning app and can contain tools such as history, notes, or softphone. Favorites, home page, and global actions are not solutions that provide access to components from anywhere within the app. See [this article] for more information on Utility bar.

For Universal Containers to understand the return on investment (ROI) for their latest advertising buy while using a private security model, the best recommendation is to use Campaign Hierarchies and Campaign statistics.
Campaign Hierarchies and Campaign Statistics: Campaign hierarchies allow Universal Containers to track the effectiveness and ROI of marketing efforts by grouping related campaigns (e.g., advertising buys) under a parent campaign and rolling up key metrics like responses, converted leads, and won opportunities. Campaign statistics provide detailed insights into individual and aggregate campaign performance, helping calculate ROI.
This method works well within a private security model since it respects existing sharing rules and access controls, ensuring that only authorized users can see the campaign data.
Campaign Management Overview
Option A (Account Hierarchies and Roll-Up Summary fields): Account hierarchies and roll-up summary fields focus on aggregating account-level data, which doesn't directly support measuring the ROI of advertising campaigns.
Option B (Opportunities Pipeline Report): While an opportunities pipeline report can provide insights into sales, it doesn't track campaign-related data or advertising ROI directly.
Option C (Change to a Public Security Model): Changing to a public security model isn't necessary and could expose sensitive data. Campaign hierarchies and statistics work well with a private security model.
In summary, Campaign Hierarchies and Campaign Statistics are the recommended approach to track the ROI of advertising buys.