Which of the following is a main contribution type allowed in a 401(k) plan?

Salary deferral contributions are indeed a primary type of contribution allowed in a 401(k) plan. This refers to the portion of an employee’s salary that they elect to defer and contribute to their retirement plan before taxes are taken out, which can significantly increase the employee's long-term savings. This type of contribution is incentivized by allowing participants to grow their investments tax-deferred until they withdraw the funds, typically during retirement.

The other options provided do not constitute contributions to a traditional 401(k) plan. Health insurance premiums, while an important aspect of employee benefits, are not classified as contributions to a retirement plan and are instead deducted from an employee's salary for health coverage. Life insurance premiums are similar; while they may be offered as part of an employee benefits package, they do not contribute to a 401(k). Severance pay is a form of departure compensation rather than a contribution type, and while it can sometimes be rolled over into retirement accounts, it is not itself a standard contribution made to a 401(k) plan.