\(v\)rms=\(\sqrt{\frac{3RT}{M_0}}\)
T is doubled and oxygen molecule is dissociated into atomic oxygen molar mass is halved.
So, \(v'\)rms=\(\sqrt{\frac{3R×2T}{M_0/2}}\)=2\(v\)rms
Therefore, the correct option is (B): velocity of atomic oxygen is doubled.