Final answer:
Asteroids are predominantly composed of iron and carbon, with the carbonaceous (C-type) asteroids being rich in carbon and the stony (S-type) asteroids containing silicates but lacking carbon compounds.
Step-by-step explanation:
Asteroids are mostly made up of a variety of elements, but when looking at common compositions, two elements that frequently appear are iron and carbon. C-type asteroids ("C" for carbonaceous) contain significant amounts of carbon compounds and silicates, providing them with a darker appearance in spectral studies. These C-type asteroids represent some of the more chemically primitive objects in our solar system, having changed little since their formation. Meanwhile, S-type asteroids ("S" for silicate or stony) are made of minerals that contain silicon but are notably missing the dark carbon compounds found in C-type asteroids. While iron is not explicitly stated in this context, it's common knowledge that many asteroids also have substantial amounts of iron and nickel, making 'a) Iron' and 'd) Carbon' the correct answers to the question.