Code 7B is correct.
Because the Roth 401(k) is not yet qualified, the distribution is a proportionate mix of nontaxable contribution basis and taxable earnings. Box 1 will show the entire gross amount distributed and box 2a will show the taxable portion. The amount from box 1 will be included on Form 1040 line 5a and the amount from box 2a will be included on Form 1040 line 5b.
