A retirement plan that may be established by an employee, regardless of any other retirement plan is called a(n):

A Keogh plan is specifically designed for self-employed individuals or unincorporated businesses, allowing them to contribute toward their retirement. However, it is not universally available to all employees, as it is restricted to those in self-employment, thereby disqualifying it as a plan that can be established regardless of other retirement options.

A 401(k) plan is an employer-sponsored retirement savings plan, meaning it is available to employees only when their employer offers it. Employees cannot establish a 401(k) independently, making it unsuitable for the question's criteria of being available regardless of other plans.

Corporate pension plans are retirement plans funded and managed by employers for their employees, and they require employment at a corporation to participate. Like the 401(k), they do not allow employees to set up a plan independently, which eliminates them from being a viable choice.