If the volume of a gas is doubled at constant pressure, the average translational  kinetic energy of its molecules will

At constant pressure,

$\frac{V_1}{T_1}=\frac{V_2}{T_2}\Rightarrow T_2=\frac{V_2}{V_1}\times {\text{T}}_{\text{1}}=2{\text{T}}_{\text{1}}$

Average translational kinetic energy . $=\frac{1}{2}m{v}_{RMS}^2=\frac{3}{2}\frac{RTm}{M}\propto \left(temperature\right)$
Since, temperature doubles, average translational KE of molecules also doubles.