The primary factor responsible for delayed expansion in zinc-containing dental amalgams is moisture contamination during trituration.

- Zinc is added to dental amalgam alloys to improve their properties by minimizing oxidation of other elements during storage.
- However, zinc reacts adversely with moisture, especially when moisture contamination occurs during the *trituration* (mixing) phase. When moisture contacts the zinc-containing alloy, a hydrogen gas is generated due to a reaction between moisture and zinc.
- This gas gets trapped within the amalgam mass, causing delayed and excessive expansion.

Let's analyze why the other options are incorrect:
- Improper mixing ratio of alloy to mercury (Option 1): While incorrect ratio affects the final consistency and working properties of the amalgam, it is not the primary cause of delayed expansion.
- Use of high-copper alloy instead of low-copper alloy (Option 3): High-copper alloys are actually preferred today to reduce issues like marginal breakdown and corrosion; they do not cause delayed expansion.
- Excessive condensation pressure during placement (Option 4): This affects the adaptation and voids within the restoration but does not contribute to delayed expansion.

Key points to remember:
- Zinc-containing amalgams are sensitive to moisture contamination during trituration.
- Moisture reacts with zinc to produce hydrogen gas, leading to expansion.
- Care must be taken to keep all instruments and materials dry when handling zinc-containing alloys.

Reference: Sturdevant's Art and Science of Operative Dentistry, Volume 2, Chapter 15/Page 480