\(n_{MgSO_4}=1\cdot0,3=0,3\left(mol\right);n_{NaOH}=0,2\cdot1=0,2\left(mol\right)\\ PTHH:MgSO_4+2NaOH\rightarrow Mg\left(OH\right)_2\downarrow+Na_2SO_4\\ \text{Vì }\dfrac{n_{MgSO_4}}{1}>\dfrac{n_{NaOH}}{2}\text{ nên sau phản ứng }MgSO_4\text{ dư}\\ \Rightarrow n_{Mg\left(OH\right)_2}=\dfrac{1}{2}n_{NaOH}=0,1\left(mol\right)\\ \Rightarrow m_{Mg\left(OH\right)_2}=0,1\cdot58=5,8\left(g\right)\)
Chọn B