Final answer:
Calcium is not one of the four most abundant elements in the Earth's crust; it ranks fifth. The four most abundant elements are oxygen, silicon, aluminum, and iron.
Step-by-step explanation:
The four most abundant elements in the Earth's crust are oxygen, silicon, aluminum, and iron. Looking at the options provided, calcium is not one of these top four but is actually the fifth most abundant element on Earth. Therefore, the element that is not one of the four most abundant in the Earth's crust is calcium. Oxygen makes up 47 percent by weight of the Earth's crust, while silicon accounts for 28 percent by weight. Together, oxygen and silicon form a major part of silicates which estimate for about 90% of the Earth's crust. Aluminum and iron also contribute significantly to the crustal composition. In contrast, elements like hydrogen and nitrogen are more abundant in the Earth's biosphere rather than its crust.