Hi @Anagha Karurkar and welcome to Codewars;

As you can see by the 3 kyu rating, this kata is intended to be a difficult one so it's hard to give you tips/hints without spoiling, especially without knowing what your level is in mathematics/computer science.

If you are new to this area, my recommendation would be to search the Codewars catalogue for the keyword "Fibonacci" - there are 9 other katas in C# that are ranked easier than this one.

By solving them, from easiest to hardest, you should gradually learn about some more and more efficient approaches and maybe pick up some ideas along the way. Then you can return to this one later.

I know loops is bad option when it comes to big numbers.
Tried to use golden ratio . Still not correct answer.
Using c# language to solve this kata.
Does anyone know any other efficient way/ formula?