Social Security is closer to a

multiple choice

defined contribution plan, because if you participate for the required period of time, you make a contribution according to a set of rules and a formula.

defined benefit plan, because those enrolled contribute to an account according to a formula, which can be $100$ percent employee, $100$ percent employer, or some mix.

defined benefit plan, because if you participate for the required period of time, you get a benefit according to a set of rules and a formula.

defined contribution plan, because if you participate for the required period of time, you get a benefit according to a set of rules and a formula