**Core Concept**
The transplantation of a cornea is a delicate procedure that requires precise timing to ensure optimal graft survival and minimize complications. The cornea's avascularity and transparency are essential for clear vision, making the timing of the transplant critical.

**Why the Correct Answer is Right**
The correct answer is 7 days. This is because the cornea's endothelial cells, responsible for maintaining corneal clarity, are most viable within the first 7 days after death. After this period, the endothelial cells undergo significant loss, leading to decreased graft clarity and increased risk of postoperative complications. The endothelial cell count is a crucial factor in determining the suitability of a cornea for transplantation.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a timeframe, making it an ambiguous choice.
**Option B:** This option is incorrect because it is too vague, as the optimal timeframe for corneal transplantation is not within a few weeks.
**Option C:** This option is incorrect because the cornea's viability does not decrease significantly after 7 days, making it an incorrect assertion.

**Clinical Pearl / High-Yield Fact**
When considering corneal transplantation, it is essential to assess the donor cornea's endothelial cell count within 7 days of death. This helps determine the graft's potential for clear vision and minimizes the risk of postoperative complications.

**Correct Answer: D. 7 days**