a
The balanced chemical reaction is shown below:

\(M g+\frac{1}{2} O_{2} \rightarrow M g O\)

Moles: \(\frac{1.0}{24} ; \frac{0.56}{32}\)

\({\frac{0.5}{12} ; \frac{0.07}{4}}\)

\({\frac{0.5}{12}-x ; \frac{0.07}{4}-\frac{x}{2}}\)

Oxygen is limiting reagent so, \(\frac{0.07}{4}-\frac{x}{2}=0\)

\(x=\frac{0.07}{2}\)

Excess \(M g=\frac{0.5}{12}-\frac{0.07}{2} \;\mathrm{mol}\)

Mass of \(M g\) is \(=1-0.7 \times 12=0.16\; \mathrm{g}\)

Thus, when \(1.0 \;g\) of magnesium is burnt with \(0.56 \;\mathrm{g} O_{2}\) in a closed vessel, \(0.16\; \mathrm{g}\) magnesium is left in excess.