The composition of Earth's core is mostly iron. To determine this, scientists have used a variety of methods, including seismic observations and studies of meteorites. By studying the behavior of seismic waves as they pass through the Earth, scientists can infer the properties of its layers, including the core. Additionally, the study of meteorites, which are believed to be remnants of early planetary formation, provides valuable insights into the composition of Earth's core. These studies have led to the conclusion that Earth's core is predominantly composed of iron, with smaller amounts of other elements such as nickel and sulfur. Therefore, the correct answer is "mostly iron".
