Universal Container’s Salesforce admin has modified the page layouts for accounts and opportunities in the lightning experience. Where would the fields appear when the users access the account or the opportunity page layout?

Answer options:

A.Fields will appear under the highlight panel
B.Fields will appear under the related tab
C.Fields will appear under the details tab
D.Fields will appear under the activity tab

Correct Answer: C 
The Detail tab contains all the fields that are available on the page layout in the lightning experience. 
Option A is incorrect because the highlight panel shows the high-level fields that are customized on the compact layout. 
Option B is incorrect because the related tab will show the related list information. 
Option D is incorrect because the activity tab shows everything related to activities. 
Reference: 

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