In the app manifest for your Microsoft Teams app you'll define a single messaging extension with up to ten different commands. Each command defines a type (action or search), and the locations in the client it can be invoked from (compose message area, command bar, and/or message).
Reference: https://docs.microsoft.com/en-us/microsoftteams/platform/messaging-extensions/what-are- messaging-extensions

Office UI Fabric is Microsoft's official front end framework for building User Interface experiences for Office and Office 365. In simple terms it gives you the look and feel for your component. Moreover, they are open source, mobile responsive and reusable, no need to create from scratch just refer them in your code and start utilizing them.
Microsoft modern SharePoint out of the box webparts are made up these Office UI Fabric framework, so to maintain consistency look and feel as like Office it's better to use these components in our custom SPFx webparts.
Incorrect Answers:
D: At Build 2019 [November 2019], Microsoft announced Fluid Framework, a new technology and set of experiences that will make collaboration seamless by breaking down the barriers between apps. It offers three key capabilities. First, experiences powered by the Fluid Framework will support multi-person coauthoring on web and document content at industry-leading speed and scale. Second, it provides a componentized document model that allows authors to deconstruct content into collaborative building blocks, use them across applications, and combine them in a new, more flexible kind of document. Third, the Fluid Framework makes room for intelligent agents to work alongside humans to translate text, fetch content, suggest edits, perform compliance checks, and more.
Reference: http://www.sharepointsamples.com/sharepoint-framework-webpart-with-office-ui-fabric-react- component/

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Explanation
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Reference:
https://medium.com/netcompany/custom-microsoft-teams-applications-the-easy-way-6da0a5975336