The correct option is C 360 J
We know,
root mean square velocity=√3RTM
Where,
M=molar mass in kg
T=temperature
R=gas constant
∴300=√3RT4×10−3
⇒RT=120
Again, we know, total K.E. of He gas =32nRT
Here number of moles of He = 84
∴K.E of He gas=32×84×120 J
=360 J