Step 1: Understand the question.
At the same temperature and pressure, on the same adsorbent, we must find which gas is adsorbed the most.
Step 2: Recall the key rule.
For physical adsorption, the gas that is easiest to turn into a liquid is adsorbed the most. Ease of liquefaction is shown by the critical temperature.
Step 3: Link to critical temperature.
A higher critical temperature means stronger forces between the gas molecules, so it sticks to the surface more easily.
Step 4: Compare the gases.
The critical temperatures are roughly: $Cl_2$ about 417 K, $O_2$ about 154 K, $N_2$ about 126 K, $H_2$ about 33 K.
Step 5: Pick the highest.
Chlorine has the highest critical temperature, so it liquefies most easily and is adsorbed the most.
Step 6: Choose the answer.
The gas adsorbed most is $Cl_2$, which is option 2.
\[ \boxed{Cl_2} \]