What is a 401(C) plan?

I believe you may have a typo in your question. The correct term is a 401(k) plan, which is a tax-advantaged retirement savings account offered by employers in the United States. Employees can contribute a portion of their pre-tax income to the plan, which is then invested in a variety of options such as stocks, bonds, and mutual funds. The contributions grow tax-deferred until withdrawal during retirement, at which point they are taxed as ordinary income.