High and Low method

The high-low method is only relevant for semi-variable costs!! It is irrelevant when costs of purely variable or are purely fixed (even if stepped fixed).

yes, but going through Kaplan they have mentioned the methods for variable cost and stepped fixed

I think that you have misunderstood what the book has written. The whole purpose (and the only purpose) of the high/low method is to separate out the variable cost per unit and the total fixed cost when looking at semi-variable costs. I suggest that you watch my free lectures on this. The lectures are a complete free course for Paper MA and cover everything needed to be able to pass the exam well. If you are watching the lectures then you do not really need the Study Text. The book that is essential, regardless of how you choose to study, is the Exam Kit because it is full of exam standard questions for practice, and practice is vital to passing the exam.

surely sir just rewatched the lecture now got my doubt cleared. thank you for your help sir

You are welcome :-)