B. A top-level class can only have one inner class level. is correct
"You can only have inner classes one level deep."

D. Classes are final by default. is correct : "The private access modifier declares that this class is only known locally, that is, only by this section of code. This is the default access for inner classes."

"Methods defined within an interface have no access modifiers and contain just their signature."
B and D.

Also D: https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_keywords_final.htm