The correct answer is option 3. Iron.
A compound is a substance composed of two or more different elements chemically bonded together in fixed proportions. Let's analyze each option:
1. Sodium Chloride: This is a compound composed of sodium (Na) and chlorine (Cl) atoms bonded together in a 1:1 ratio to form the compound \(NaCl\), which is commonly known as table salt.
2. Carbon Monoxide: This is a compound composed of carbon (C) and oxygen (O) atoms bonded together in a 1:1 ratio to form the compound \(CO\). It is a toxic gas produced by incomplete combustion of carbon-containing fuels.
3. Iron: Iron is not a compound; it is a chemical element with the symbol Fe. It exists as individual iron atoms and is not chemically bonded to other elements in the context of this question.
4. Water: This is a compound composed of hydrogen (H) and oxygen (O) atoms bonded together in a 2:1 ratio to form the compound \(H_2O\). It is a well-known substance essential for life.
Therefore, option 3. Iron is not a compound. It is an element | 
