Final answer:
Oxygen is the element that is a gas at 25°C and 1 atm; the other options, iron, carbon, and bromine, are not in the gaseous state under these conditions.
Step-by-step explanation:
The element that is a gas at 25°C and 1 atm from the options given is B) Oxygen. At room temperature and atmospheric pressure, certain elements occur naturally as gases, including noble gases like helium and neon, and diatomic molecules such as hydrogen (H₂), nitrogen (N₂), oxygen (O₂), fluorine (F₂), and chlorine (Cl₂).
Looking at the other options, iron is a solid metal, carbon is a solid in its most common forms (such as graphite or diamond), and although bromine is close to being a gas, it is actually a liquid at room temperature. Hence, the correct answer is oxygen which exists as a diatomic molecule, O₂.