Final answer:
Magnesium is excreted by the kidneys and can be lost through the gastrointestinal tract. The excretory system includes other organs like the skin and lungs, which also play a role in waste removal.
Step-by-step explanation:
The question asks about the excretion of magnesium in the human body. Magnesium is excreted by the kidneys, but it can also be lost through other ways, such as the gastrointestinal tract. Therefore, the correct answer to the question is a) Kidneys, gastrointestinal tract.
The kidneys are primarily responsible for the urinary excretion of calcium and other minerals like magnesium, maintaining the balance of electrolytes in the body. The gastrointestinal tract, on the other hand, can lose magnesium through feces. Other organs that are part of the excretory system, such as the skin and lungs, also have roles in excreting waste products like salts and carbon dioxide, respectively.