Which variable cannot be modified inside lambda expression

Question | Jul 22, 2017 | nextptr 

A lambda expression is created and invoked inside method foo() of class Inner, which is an inner class of class Outer:

// import java.util.function.*; 
class Outer {

  int a = 1; // Outer field a

  class Inner {

   int b = 2; // Inner field b

   void foo() {  // Inner method foo

    int c = 3; // Local variable c

    // create lambda 
    Consumer<Integer> r = (x) -> {
     /* Modify x, c, b, a */
      x++; c++; b++; a++; // ERROR!!

    // invoke lambda 

   } // foo end

 } // Inner end

} // Outer end

The lambda expression refers to 3 variables from its outer scope: local variable c of foo, field b of Inner, and field a of Outer. The lambda expression itself takes an integer parameter x.

Above code fails to compile because one of x, c, b, and a cannot be modified inside the lambda. Which one of the 4 variables you think cannot be modified inside the lambda expression?