What of these is referred to as the "third rail" of american politics
a. the aging of population
b. medicaid
c. medicare
d. social security

The right answer is the D) Social Security.

Explanation: Just to elaborate a little on the answer, it can be added that the expression "third rail"— a reference to the third rail system, a method of providing electric power to a locomotive by means of an additional rail that can cause electrocution if someone touches it — is used in politics in order to refer to a highly controversial topic, which, if tackled by a politician, will certainly have a long-lasting effect (usually detrimental) on his or her career. Arguably coined by Kirk O'Donnell, assistant to the Democratic speaker of the House, Thomas O'Neill, during the Reagan administration, the full quote read: "Social Security is the third rail of American politics. Touch it, you're dead."