The correct answer is A. Graphite.
Graphite is a form of carbon that is used as a lubricant because it is soft and slippery. It is also a good conductor of heat and electricity, which makes it useful in a variety of applications.
Silica is a type of sand that is not used as a lubricant. It is hard and abrasive, which would actually damage the surfaces that it is supposed to be lubricating.
Iron oxide is a type of rust that is also not used as a lubricant. It is also hard and abrasive, and it would not provide any benefit as a lubricant.
Diamond is the hardest natural substance on Earth. It is also very brittle, and it would not be able to withstand the pressure and heat that is generated when two surfaces rub together. Therefore, diamond is not used as a lubricant.