Which one is correct?

Question

In order to prevent duplicate recordset in child class, we..
a. Define base class as virtual (multiple inheritance.
b. define virtual function in the base class
c. make or define Virtual functions in the base class.
d. All option is correct.

Note: please tell me the difference of option B and C? I got this question fee Times. At first I choose option D and sololearn flagged it as right or correct. Latest, eventhough I choose option D as my answer, sololearn flags my answer as false:) could you or sololearn please explain me?

Answer 1

In common sense all the four options are correct but if it's a quiz then the answer may have been changed to the most appropriate option

Answer 2

Make constructor of base class as virtual i think..
Option A..