You need to ensure that the right code is present in the mobile component to initialize App Center. 

Which of the following would go into Slot1?

Answer options:

A.MSAnalytics.self
B.MSDistribute.self
C.MSCrashes.self
D.MSPush.self

Answer - A 
An example of this is given in the Microsoft documentation. 

Since this is clearly given in the documentation, all other options are incorrect. 
For more information on using App Center with iOS apps, please visit the below URL- 

https://docs.microsoft.com/en-us/appcenter/sdk/getting-started/ios