The correct answer is: D. Stainless steel
Stainless steel is a type of steel that is resistant to corrosion. It is made by adding chromium to the steel, which forms a protective layer on the surface of the metal. This layer prevents the steel from rusting or corroding, even when it is exposed to moisture or other elements.
Stainless steel is a good choice for precision levelling staff because it is strong and durable. It is also resistant to wear and tear, which makes it ideal for use in harsh environments.
The other options are not as suitable for precision levelling staff. Titanium steel is a type of steel that is strong and lightweight. However, it is not as resistant to corrosion as stainless steel. Carbon steel is a type of steel that is strong and affordable. However, it is not as resistant to corrosion as stainless steel. Invar is a type of steel that is very stable and has a low coefficient of thermal expansion. However, it is not as strong or durable as stainless steel.
Therefore, the best type of steel for precision levelling staff is stainless steel.