Which metal is least likely to corrode?

Titanium is least likely to corrode due to its unique chemical properties and the formation of a protective oxide layer. When exposed to oxygen, titanium readily forms titanium dioxide on its surface, which acts as a barrier to further oxidation and corrosion. This oxide layer is not only stable and adherent but also highly resistant to corrosion in a variety of environments, including saltwater and acidic conditions.

In contrast, while iron, aluminum, and copper can have varying levels of resistance to corrosion, they are generally more susceptible to oxidation and corrosion processes. Iron, for example, begins to rust quickly when exposed to moisture and air. Aluminum can corrode under certain conditions as well, but it too develops a thin oxide layer which helps protect it to some extent. Copper, while generally resistant to corrosion and developing a patina, can still be affected by environmental factors over time. Therefore, titanium is the metal that stands out for its superior resistance to corrosion, making it the correct answer in this context.