C. How many molecules are in 0.400 moles of N205?

2.408x10^23 molecules

Explanation:

Avogadro's hypothesis gives us a background understanding that 1mole of any substance contains 6.02x10^23 molecules.

From the above conclusion,

1mole of N205 contains 6.02x10^23 molecules.

Therefore, 0.4mole of N2O5 will contain = 0.4 x 6.02x10^23 = 2.408x10^23 molecules