b
\(\begin{array}{l}
\,\,\,{\rm{4AI}}\,\,\,\, + \,\,\,\,{\rm{3}}{{\rm{O}}_{\rm{2}}}\, \to \,\,2A{l_2}{O_3}\\
\,\,\,\,\,\,\,\,\,\,\,\,\,\,\frac{4}{3} \times \frac{1}{2}\,\,\,\,\,\,\,\,\,\frac{1}{2}\\
વજન \,\,\frac{{\rm{2}}}{{\rm{3}}} \times 27 = 18\,g
\end{array}\)