Answer: B. The molar volume is $V/n = RT/P$ Both $T$ and $P$ double, so $T/P$ and the molar volume remain unchanged; with the amount fixed, the volume also remains unchanged.
A: considers the pressure increase but ignores the effect of the doubled temperature.
C: considers the temperature increase but ignores the effect of the doubled pressure.
D: multiplies the temperature and pressure factors instead of using their ratio.
Remember: at fixed amount, volume changes in the same proportion as molar volume.