Answer
$1.527Kg \space Mn$
Work Step by Step
We can find the mass of $Mn$ as
$1\times 10^3g\times \frac{1mol \space Al}{26.98g\space Al}\times \frac{3 mol Mn}{4 mol Al}\times \frac{54.94g\space Mn}{1 mol \space Mn}=1.527\times 10^3\space Mn=1.527Kg \space Mn$