The inability to create a Roll-Up Summary field in this context can be attributed to:
* B. The organization has Advanced Currency Management enabled. When Advanced Currency Management (ACM) is enabled, it affects how currency data is managed and aggregated, impacting the functionality of roll-up summary fields that involve currency amounts across different records.
To address this limitation, consider using custom reporting or a custom solution like a trigger or a scheduled batch class to manually calculate and update the total.
For more information on limitations with ACM, consult Salesforce's help article on Advanced Currency Management.

To connect Testimonial__c records with both the submitter's Contact record and Account record, an app builder should create a master-detail relationship from Testimonial__c to Account and a lookup relationship from Testimonial__c to Contact. A master-detail relationship creates a parent-child relationship between two objects and allows for roll-up summary fields, sharing settings, and cascading actions. A lookup relationship creates a loose association between two objects and allows users to relate records from one object to another. Source

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.

The app builder should provision a Partial Data sandbox to the developer. A Partial Data sandbox can have up to 5 GB of data, which meets the requirement of having at least half a gigabyte of data. A Partial Data sandbox can also be refreshed every 5 days, which meets the requirement of refreshing after each three-day sprint. Option A is incorrect because a Developer sandbox can only have up to 200 MB of data, which does not meet the requirement. Option B is incorrect because a Full Copy sandbox can only be refreshed every 29 days, which does not meet the requirement. Option C is incorrect because a Developer Pro sandbox can only have up to 1 GB of data, which may not be enough for the requirement.

A roll-up summary field is a field that calculates values from related records. A roll-up summary field can be used to display the number of opportunities that are closed won with a close date within the last year on the account detail page by using a filter criteria on the field.