Answer:
Option D. 400 mmHg 
Explanation:
The following data were obtained from the question:
Mole of He (nHe) = 0.04 mole
Mole of Ne (nNe) = 0.06 mole
Total pressure = 10³ mmHg
Partial pressure of He =.? 
Next, we shall determine the total number of mole in the reaction vessel.
This can be obtained as follow:
Mole of He (nHe) = 0.04 mole
Mole of Ne (nNe) = 0.06 mole
Total mole =? 
Total mole = nHe + nNe
Total mole = 0.04 + 0.06
Total mole = 0.1
Next, we shall determine the mole fraction of He. 
This can be obtained as follow:
Mole fraction = mole of gas /total mole 
Mole of He (nHe) = 0.04 mole
Total mole = 0.1
Mole fraction of He =.? 
Mole fraction of He = nHe/total mole 
Mole fraction of He = 0.04/0.1
Mole fraction of He = 0.4
Finally, we shall determine the partial pressure of He as follow:
Partial pressure = mole fraction x total pressure 
Mole fraction of He = 0.4
Total pressure = 10³ mmHg
Partial pressure of He =.? 
Partial pressure of He = 0.4 x 10³
Partial pressure of He = 400 mmHg. 
Therefore, the partial pressure of He is 400 mmHg.