Answer
a. Line A corresponds to $Cl_{2}$, and line B corresponds to $He$.
b. Line A corresponds to 273 K and line B corresponds to 1273 K. $O_2$ will behave most ideally at 1273 K.
Work Step by Step
a. Because $Cl_2$ has a greater mass than $He$, a larger fraction of $Cl_2$ molecules will possess a lower velocity than $He$ which corresponds to Line A. Similarly, because $He$ is lighter than $Cl_2$, a greater fraction of $He$ molecules is able to reach higher velocities which correspond to Line B.
b. At higher temperatures, a greater fraction of molecules is able to reach higher velocities. Therefore, Line A corresponds with 273 K as a greater fraction of molecules is found at a lower velocity. Similarly, Line B corresponds with 1273 K as a greater fraction of molecules, relative to at 273 K, is able to reach higher velocities.
$O_2$ will behave most ideally at higher temperatures such as 1273 K because at higher temperatures, $O_2$ molecules experience a greater average kinetic energy and velocity. As a result, because of this higher energy and velocity, intermolecular forces between $O_2$ molecules are minimized at higher temperatures which is required for a gas to act ideally.
