Final answer:
The gas with the highest rms speed, which results in the highest average molecular speed at 298K, is oxygen.
Step-by-step explanation:
The highest average molecular speed at 298K can be determined by analyzing the Maxwell-Boltzmann speed distribution of different gases. According to the Maxwell-Boltzmann distribution, the most probable speed is slightly less than the root mean square (rms) speed. Therefore, to determine the gas with the highest average molecular speed, we should look for the gas with the highest rms speed at 298K.
In the provided information, we are given the most probable speed for oxygen molecules at 296 K, which is 263 m/s. Assuming that the oxygen gas has a higher rms speed at 298K than the given most probable speed, we can conclude that oxygen molecules have the highest average molecular speed at 298K.
It's important to note that the average molecular speed can vary depending on the temperature and the specific gas being considered.