What Does It Mean When Social Security Is Called the Third

What does it mean when Social Security is called the third rail of politics?

A) It is a program that spends more than it generates by way of revenue.
B) It is a program that generates more revenue than it is spending.
C) Politicians propose frequent changes to it in order to appease their constituents.
D) Politicians do not like to propose changes because they would be unpopular with voters.