The correct answer is option 3: 1 and 2.

When iron is left exposed in open air, it undergoes a process called oxidation, which leads to rust formation. This process is facilitated by the presence of both oxygen gas and moisture in the air.

Option 1 states that oxygen gas present in the air is responsible for rusting iron. This is true because oxygen participates in the chemical reaction that results in the formation of rust. It reacts with iron to form iron oxide, which is the reddish-brown substance we commonly refer to as rust.

Option 2 mentions moisture present in the air. This is also accurate because the presence of water (in the form of moisture) is essential for rusting to occur. Water molecules react with iron and oxygen to create iron hydroxide, which eventually converts into rust.

Therefore, both oxygen gas and moisture play vital roles in rusting iron, making option 3 the correct answer.