Final answer:
The temperature at which O₂ has the highest velocity is 325 °K.
Step-by-step explanation:
The temperature at which O₂ has the highest velocity can be determined using the concept of kinetic energy and the molecular speed distribution. According to the given information, the most probable speed for O₂ molecules at 300 K is a little less than 400 m/s, while the root mean square speed is closer to 500 m/s. As the temperature increases, the kinetic energy of the molecules also increases, leading to an increase in their velocities. Therefore, the temperature at which O₂ has the highest velocity would be higher than 300 K.
Based on the given options, the temperature closest to or higher than 300 K is 325 °K (option E). Therefore, option E, 325 °K, is the temperature at which O₂ has the highest velocity.