The two decay products must be back to back in order to conserve three momentum (the initial three momentum is ). Let be the four-vector of the massless particle. Equation (1) with replaced by implies that . So, the four-momentum of the massless particle is . By three-momentum conservation, the three-momentum of the massive particle must be . From equation (1), the four-momentum must then be . The initial energy is , so by energy conservation,
Squaring both sides and solving for yields
Therefore, answer (B) is correct.