During arc welding the molten weld pool is chemically very reactive. If it is exposed to the surrounding air, gases such as oxygen, nitrogen and hydrogen dissolve into the liquid metal. To prevent this, the pool is protected by a shielding medium — an inert or active gas in MIG/TIG welding, or the gas released by a burning flux coating in stick (SMAW) welding — which blankets the arc and keeps the atmosphere out.

When this shielding is inadequate (too high a gas flow causing turbulence, too low a flow, wind blowing the shield away, or a damaged flux coating), atmospheric gases enter the pool. As the weld solidifies its ability to hold dissolved gas drops sharply, so the gas comes out of solution and gets trapped as bubbles. Once the metal freezes around these bubbles they remain as rounded voids or gas pockets in the weld — the defect called porosity.

• Other common causes of porosity include moisture on the workpiece or electrode, oil, rust and other surface contamination, and an excessively long arc — all of which introduce or generate gas.

The distractors are genuine weld defects but arise from different mechanisms. Slag inclusion is solid, non-metallic flux/slag trapped in the weld, usually from poor inter-pass cleaning, not from shielding-gas failure. Undercut is a groove melted into the base metal at the toe of the weld caused by excessive current, heat or a wrong travel angle — a geometric defect, not a trapped gas. Lack of fusion is failure of the weld metal to bond with the base metal due to insufficient heat input or poor technique. Only porosity is specifically the gas-pocket defect produced by improper shielding, so it is the correct answer.