Which compound can act as a Lewis base by forming a complex with trialkylamine?

The compound sulfur dioxide can act as a Lewis base because it has a lone pair of electrons on the sulfur atom that can be donated to form a coordinate covalent bond with a Lewis acid, such as trialkylamine. In Lewis acid-base theory, a Lewis base is defined as a species that can donate a pair of electrons to form a bond, and sulfur dioxide fits this definition due to its electron-rich sulfur center.

In the case of complex formation with trialkylamine, the sulfur's lone pair interacts with the electron-deficient site of the amine, facilitating the formation of a stable complex. This interaction demonstrates the Lewis base characteristic of sulfur dioxide effectively.

Other options like sulfur trioxide, sodium chloride, and carbon dioxide do not possess the necessary properties to act as Lewis bases in this context. Sulfur trioxide, for instance, does not have a lone pair available for donation and generally acts as a Lewis acid due to its electron-accepting capabilities. Sodium chloride is an ionic compound that doesn't exhibit Lewis base behavior, and carbon dioxide, while it can participate in some reactions as a Lewis acid, does not have the same ready capability to form a complex with trialkylamine compared to sulfur dioxide.