$n_{MgO}=\dfrac{4}{40}=0,1mol$
$a.MgO+H_2SO_4\to MgSO_4+H_2O$
$\text{b.Theo pt :}$
$n_{H_2SO_4}=n_{MgO}=0,1mol$
$⇒m_{H_2SO_4}=0,1.98=9,8g$
$⇒C\%_{H_2SO_4}=\dfrac{9,8}{200}.100\%=4,9\%$
$\text{c.Theo pt :}$
$n_{MgSO_4}=n_{MgO}=0,1mol$
$⇒m_{MgSO_4}=0,1.120=12g$
$m_{dd\ spu}=4+200=204g$
$⇒C\%_{MgSO_4}=\dfrac{12}{204}.100\%≈5,88\%$