Final answer:
The Earth's crust is primarily composed of the elements silicon and oxygen. Oxygen makes up about 46.1% and silicon about 28.2% by weight of the crust, combining to form various silicate minerals which constitute around 90% of the crust.
Step-by-step explanation:
The question 'Which of the following pairs of elements composes most of the Earth's crust?' pertains to the composition of the Earth's crust. When examining the composition of the Earth's crust, we find that silicon and oxygen are the two most abundant elements. Oxygen is the single most abundant element, accounting for about 46.1% by weight of the crust, primarily combining with other elements, while silicon contributes to about 28.2% by weight.
Together, these two elements form various silicate minerals, which constitute a huge portion of the Earth's crust. Around 90% of the Earth's crust is made up of silicates, which are SiO₄-based materials. The abundant presence of these elements and their compounds also impacts the formation of rocks and minerals on the terrestrial planets, which includes Earth. These facts solidify that the correct answer to the student's question is a. silicon and oxygen.
Iron and nickel (b. iron and nickel), though significant components of the Earth's core, are less abundant in the crust compared to silicon and oxygen. Iron does contribute to the crust's composition but not as predominantly as silicon and oxygen. Nitrogen and oxygen (d. nitrogen and oxygen), while prevalent in the atmosphere, with oxygen being a major component of the crust as well, nitrogen is not as abundant in the crustal composition. Similarly, calcium, magnesium, iron, and carbon (a. calcium, magnesium, iron, and carbon) do not account for the most abundant pair when considering the Earth's crustal composition. Thus, it is evident that the majority of the Earth's crust is composed of silicon and oxygen.