Option 1: Tin is a pure element and not a mixture. It is made up of atoms of a single type, which means it has a uniform composition throughout.

Option 2: Seawater is a mixture. It is composed of various substances such as water, salt, dissolved gases, and minerals. These components can vary in concentration and can be physically separated.

Option 3: Soil is also a mixture. It consists of a combination of minerals, organic matter, water, and air. The composition of soil can vary depending on its location and can be separated into its individual components.

Option 4: Air is a mixture of gases. It is composed of primarily nitrogen, oxygen, carbon dioxide, and other trace gases. The proportions of these gases can vary, making air a mixture.

In summary, all options except for option 1 are mixtures. Tin is the correct answer as it is a pure element, not a mixture.