Which type of pension plan allows benefits to begin at age 55 with 15 years of service?

The correct answer highlights a specific feature associated with defined benefit plans, which is their promise of benefits based on a formula usually involving years of service and salary history. Defined benefit plans are often structured to provide retirement income to participants after a certain age, or after a designated number of service years.

In this case, the stipulation that benefits can commence at age 55 with 15 years of service aligns well with the characteristics of a defined benefit plan, as these plans typically allow for early retirement options, enabling participants to access their benefits earlier than the standard retirement age, provided they meet the specific service requirements.

This early retirement provision is less common in defined contribution plans, which are primarily based on individual contributions and investment performance. Nonqualified retirement plans are typically designed for select employees and may not offer the same clear age and service criteria for benefit commencement. Solo 401(k) plans are individual retirement accounts set up for self-employed individuals; they do not typically include benefits structured around years of service or age of retirement in the manner defined benefit plans do.

Thus, the correct answer reflects how defined benefit plans ensure a defined stream of income based on the criteria mentioned in the question.