Retirement plans generally fall into two categories: defined benefit and defined contribution. A defined contribution plan (Option B), such as a 401(k) or 403(b), establishes an individual account for each employee. The ultimate retirement benefit is not a fixed amount but is instead based on the total amount contributed to the account (by both the employee and employer) and the investment gains or losses on those funds. This shifts the investment risk from the employer to the employee. In contrast, a defined benefit plan (pension) promises a specific monthly benefit at retirement based on a formula (usually salary and years of service), where the employer is responsible for ensuring enough funds are available. Section 125 (Option D) refers to pre-tax health and welfare benefits, not a retirement vehicle.