Corrosion resistance in steels depends heavily on whether the alloy can form a passive protective film on its surface. Ordinary mild steel is essentially iron with a low carbon content and no significant amounts of the alloying elements that create such a film.

Why the absence of protective elements like chromium or nickel is correct:

• Mild steel lacks chromium, nickel and other passivating elements. In stainless steel, chromium (typically ≥ 10.5%) reacts with oxygen to form a thin, dense, adherent and self-healing chromium-oxide (Cr₂O₃) layer that seals the surface and stops further attack.
• Without chromium, mild steel instead forms iron oxide (rust, largely Fe₂O₃·xH₂O). This oxide is porous, flaky and non-adherent, so it continually spalls off and exposes fresh metal, allowing corrosion to continue unabated.

Why the other options are less correct:

• The statement that it forms a dense but non-protective oxide layer is self-contradictory — the whole problem is that the rust layer is porous and non-dense, not dense.
• A relatively high carbon content is not the primary reason; mild steel is actually low in carbon, and carbon content is not what governs passivation.
• A non-uniform microstructure can locally accelerate galvanic corrosion, but the fundamental, dominant cause of mild steel's poor corrosion resistance is the missing passivating alloy elements.