Which operator should be overloaded in the following code to make the program error free?

[answerhappygod]

#include <iostream>
#include <string>
using namespace std;
class Box{
int capacity;
public:
Box(){}
Box(double capacity){
this->capacity = capacity;
}
};
int main(int argc, char const *argv[])
{
Box b1(10);
Box b2 = Box(14);
if(b1 == b2){
cout<<"Equal";
}
else{
cout<<"Not Equal";
}
return 0;
}
a) +
b) ==
c) =
d) ()