Final answer:
Matter with a consistent and unchanging composition is known as a substance, which is either an element, consisting of one type of atom, or a compound, made up of multiple elements in a fixed ratio. Option b.
Step-by-step explanation:
Matter with a uniform and unchanging composition is known as a substance. There are two types of pure substances: elements and compounds. Elements are the simplest forms of matter that cannot be broken down by chemical means and consist of only one type of atom. Compounds, on the other hand, are pure substances that are composed of two or more elements in a constant ratio, held together by chemical bonds.
A pure substance has constant composition and properties throughout - meaning any sample of this substance you take will have the same characteristics. In contrast, a mixture comprises two or more substances that are not chemically bonded and can have variable compositions. Therefore, the correct answer to the student's question is b) Substance.