Answer: Option D
IN CHEMISTRY, A SUBSTANCE REFERS TO A MATTER WHICH HAS SPECIFIC COMPOSITION AND SPECIFIC PROPERTIES WHICH DO NOT CHANGE. A SUBSTANCE CAN BE AN ELEMENT OR A COMPOUND. A SUBSTANCE TYPICALLY HAS A CONSTANT COMPOSITION, WHICH CAN NOT BE SEPARATED BY PHYSICAL METHODS. SEAWATER AND SAND ARE NOT SUBSTANCES, BECAUSE THEY HAVE VARIED COMPOSITIONS AND THEIR COMPONENTS CAN BE SEPARATED BY PHYSICAL METHODS.