Final answer:
A compound is a pure substance that consists of two or more elements combined in definite proportions by weight, held together by chemical bonds. Water (H2O) is an example of a compound. The correct answer is (d) compound.
Step-by-step explanation:
The term that describes a substance formed by the combination of two or more elements in definite proportions by weight is called a compound. A compound is characterized by having a fixed composition and being formed from atoms of different elements that are held together by chemical bonds. For instance, water (H2O) is a compound that always consists of two hydrogen atoms bonded to one oxygen atom, maintaining this proportion in every molecule of water.
Therefore, the correct answer to the question is option (d) compound. Elements, mixtures, and compounds are all different in that an element is a pure substance that cannot be broken down into simpler substances, a mixture is a combination of two or more substances that are not chemically bonded, and a compound is a pure substance that contains two or more elements chemically combined in a definite ratio.