A B are correct

Correct options are: A and B

REASONS

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

A is correct : "Methods and classes are final by default."

According to : https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_defining.htm

B is correct: "You can only have inner classes one level deep."

According to : https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_access_modifiers.htm

C is incorrect : "Interface methods have no access modifiers. They are always global."

According to : https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_defining.htm

D is incorrect: "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."

CORRECT ANSWER IS -
C: Interface methods are public by default.
D: Inner classes are public by default.

Explanation:
C: In Apex, all methods in an interface are implicitly public, and they do not need the 'public' keyword. This is because interfaces are meant to be implemented by classes that define the behavior of the methods, and the methods must be accessible to any class that implements the interface.

D: Inner classes in Apex are public by default. This means that unless a different access modifier is specified, the inner class is accessible by any outer class in the same application namespace.

INCORRECT ANSWERS:
A: This is not correct because classes in Apex are not final by default. They can be extended unless they are explicitly defined with the 'final' keyword.

B: Incorrect because a top-level class can contain more than one inner class level. There is no such restriction in Apex that limits a top-level class to only one inner class level.