When magnesium burns in steam,the formulae of the products are

$\begin{array}{1 1}(a)\;Mg(OH)_2,H_2&(b)\;MgO,H_2\\(c)\;MgO_2,H_2&(d)\;Mg(OH)_2,O_2\end{array}$

When magnesium burns in steam,the formulae of the products are $MgO,H_2$
Hence (b) is the correct answer.