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It's arbitrary only in the sense that ##|\chi\rangle## is arbitrary. The components are completely determined by ##|\chi\rangle##. If we're asked to find the rotation operator that diagonalizes ##\rho##, then this is the answer.
I think this is a plausible interpretation of the problem: Find a unitary U such that ##U|+\rangle=|\chi\rangle##. Then find ##_B## and verify that ##[U^*]_B[\rho]_B _B## is diagonal.
That second part can be done by explicitly doing the matrix multiplication, or by proving the formula ##[\rho]_{B'}=[U^*]_B[\rho]_B _B## for arbitrary bases B and B'.
I think this is a plausible interpretation of the problem: Find a unitary U such that ##U|+\rangle=|\chi\rangle##. Then find ##_B## and verify that ##[U^*]_B[\rho]_B _B## is diagonal.
That second part can be done by explicitly doing the matrix multiplication, or by proving the formula ##[\rho]_{B'}=[U^*]_B[\rho]_B _B## for arbitrary bases B and B'.