Carbon is the correct answer because it is the molecule that is present in all of the molecules that make up all of ourselves. Carbon is the building block of life as it forms the backbone of organic compounds, including proteins, carbohydrates, lipids, and nucleic acids. It has the unique ability to form covalent bonds with other carbon atoms and a wide variety of other elements, allowing for the vast diversity of molecules necessary for life. Calcium, iron, oxygen, and ice cream are not present in all molecules that make up all of ourselves, making them incorrect options.