Final answer:
The four most common elements in the Earth's crust are oxygen, silicon, magnesium, and iron. Iron is one of the top four elements, making it the correct answer for the missing element in the student's question.
Step-by-step explanation:
The four most common elements in the Earth's crust are oxygen, silicon, magnesium, and iron. Oxygen makes up about 46.1% of the Earth's crust, with silicon contributing to 28.2%. When considering the most abundant elements, iron is one of the top four, making it the correct answer to fill in the blank. Gold, while a well-known element, is not as abundant in the Earth's crust. Elements like calcium and potassium are also common, but they do not surpass iron in abundance in the Earth's crust.