Which part of Ray Optics is more important? How do the Questions come in the Competitive exams how to face them can any one give me tips to improve in Ray Optics as well as other major topics

Everything is important in ray optics but if you are asking for most important then here you go.

Derivations - Mirror and lens formulae , refraction of prism , telescope , microscope , ,lens immersed in a liquid, Two lens in contact. (5marks)

Now for three and two marks you cannot say from which topic it will come .So,better to study everything.