doubt for inheritance please explain

Sorting your example out so it would compile gives:

class A
  {
  }
class B : A
  {
  }
class C : B
  {
  }
...
  C objc = new A();

It will still not compile: you will get an error "Cannot implicitly convert type 'MyClass.A' to 'MyClass.C'. An explicit conversion exists (are you missing a cast?)"
Because C is derived from B, and B is derived from A, each item of type C contains everything that an item of type B does, and B contains everything that class A does. What that means is that an item of class B can hold an instance of Class B or anything derived from class b. It cannot hold an instance of class A, because class A does not know anything about any additions made to create class B. If the compiler allowed you to set an instance of calss A into a class C reference variable, your program would fail at run time when you tried to use any of the class C features, because the object that the variable was referring to did not contain them.

It's a bit like saying "A textbook (class B) is a type of book (class A), and a Physics textbook (class C) is a type of textbook (class B), so if I buy a book (class A), then it will contain information on Physics, rather than poetry"

C objc=new a()
This code will not complile.

Class A is the parent of C due to inheritance hierarchy - C is a child of B and B is a child of A.
Thus A objC = new C(); will complile without any issues.

Think of it this way, subclasses can provide more than base classes, so you can easily typecast a derived object into the base class but not the other way round.