Answer is 0.10 mol.
Explanation;
n = m/M
Where n is number of moles (mol), m is the mass of substance (g) and M is the molar mass of the substance (g/mol).
n = ?
m = 0.40 g
M = 4.0 g/mol
From substitution,
n = 0.40 g /4.0 g/mol
n = 0.10 mol
Hence, the balloon has 0.10 mol of He.