Universal Container’s sales ops have asked salesforce admin to enable shared activities within Salesforce. What is true with respect to enabling shared activities?

Answer options:

A.Once shared activities are enabled, you cannot disable them
B.Once shared activities are enabled, you can disable them
C.Once shared activities are enabled, archived records will be deleted
D.Once shared activities are enabled, you cannot use them in salesforce classic

Correct Answer: A 
When you enable shared activities or if it happens to be enabled within your Salesforce org you cannot disable it. This is related to both Salesforce classic and lightning. 
Option B is incorrect because once shared activities are enabled, you cannot disable it. 
Option C is incorrect because enabling shared activities will not delete any archived records 
Option D is incorrect because enabling shared activities works the same for both salesforce lightning and classic. 

https://help.salesforce.com/articleView?id=activities_shared_considerations.htm&type=5