Final variables can only be assigned a value once, either when you declare a variable or inside a constructor. You must assign a value to it in one of these two places.

A developer is implementing an Apex class for a financial system. Within the class, the variables ‘creditAmount’ and ‘debtAmount’ should not be able to change once a value is assigned. In which two ways can the developer declare the variables to ensure their value can only be assigned one time? Choose 2 answers
 Use the final keyword and assign its value when declaring the variable.
 Use the final keyword and assign its value in the class constructor.

Its A and D. B is not the answer because Static final variables can be changed in static initialization code or where defined.
Refer:- https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_keywords_final.htm and
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_constants.htm

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

D is correct answer. A and C are wrong.
B would be correct if was Use the final keyword and assign its value in a static initializer.
Refer : https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/langCon_apex_constants.htm

It's B, D