Answer:
O2
Explanation:
The distribution of the speed of gas particles is represented via a Maxwell-Boltzmann graph which is essentially a plot of the velocity of gas particles vs the fraction of gas particles.
A proper assessment of the velocity can be made by measuring the root mean square velocity of the particles which is given as:

where, R = gas constant = 8.314 J/mol-K
T = temperature of gas in K
M = molecular weight of the gas
Based on equation (1), the velocity of the gas particle is inversely proportional to its molecular weight.
The molecular/atomic weights of the given gases are:
Ne = 20 g/mol
H2 = 2 g/mol
CO = 28 g/mol
N2 = 28 g/mol
O2 = 32 g/mol
Since O2 has the highest molecular weight, it will travel the slowest.