Shep is a public school teacher. He contributes to his employer's retirement savings plan through regular payroll deductions. His contributions are tax$-$deferred, and so are his earnings in the plan.

Shep most likely participates in which of the following types of employer$-$sponsored retirement plans?

$401($k$)$ plan.

$403($b$)$ plan.

Annuity.

Thrift Savings Plan.