There’s no denying the importance and popularity of 401(k)s for retirement savers. Americans have $4.8 trillion invested in these tax-deferred savings accounts, according to the Investment Company Institute, and there are 52 million active 401(k) participants. BrightScope, a financial-information company that rates retirement-savings plans, compiled this list for Kiplinger of the most popular mutual funds in 401(k) plans based on funds’ 401(k) assets under management.