As per question the neutralised acid is equivalent to 500ml 0.5N H2SO4.
2NH3 + H2SO4,= (NH4)2SO4
By this balanced equation we get that 1mol H2SO4 requires 2 mol or 34g NH3 for nutralisation.
1000 ml 2N H2SO4 solutiin contains 1mol H2SO4.
So 1000ml 2N H2SO4 requires 34g NH3 for nutralisation.
500ml 0.5N H2SO4 requires
(34×500×0.5)/(1000×2) g NH3
=4.25g NH3.
B