The correct option is C Air tight
Cork is an impermeable, buoyant material, a prime-subset of bark tissue that is harvested for commercial use primarily from Quercus suber (the cork oak), which is endemic to southwest Europe and northwest Africa. Cork is composed of suberin, a hydrophobic substance and because of its impermeable, buoyant, elastic and fire retardant properties, it is used in a variety of products, the most common of which is for wine stoppers. Cork's elasticity combined with its near-impermeability makes it suitable as a material for bottle stoppers, especially for wine bottles.
Cork stoppers represent about 60% of all cork based production. So, cork is an excellent material for making bottle stopper because it is air tight and not because of cheap, light or easily available.
Thus, the correct answer is option C.