How is it easy for c/c++ programmer for mastering unity's script(c#/javascript)?

Im undergraduate c/c++ programming basis, i afraid it’ll be difficult for me to master unity 's script.

I would recommend you to write in C# because it is really similar to C or C++. It wont take you a lot of time to adapt to this language.
Also, an advantage to understand Unity, but not necessary, is to have used before OpenGL, DirectX or SDL. However, this is not necessary.
Furthermore, I would recommend you to watch some tutorials.

As others say, use C#.

In some ways yo will find it familiar, in other ways not.
Ultimately, its logic is a lot more like java then like C/C++, but you can get there in
stages since you can write pretty uch pure C in it.

One thing you will have to get used to is that you don’t have pointers, you have references.
Another is that its a single inheritance + interfaces language rather then multiple inheritance.

You would probably feel more at home and it makes more logical sense that you should use C#.

From your background in C/C++ it may come more naturally to you.

Two cases,
if you are a game developer already with some knowledge about Grpahics, matrix and meshes, it would be a piece of cake for you… you can even start doing stuff from the 2nd or 3rd day.

if you are a good c++ programmer, but not a game developer, then it might take some time depending on how much effort put it to learn… either way shouldnt take more than a week or two to feel like at home.