**Core Concept:** The p53 protein is a crucial tumor suppressor protein that plays a vital role in maintaining genomic stability and preventing cancer. Its half-life determines the rate at which its concentration declines in normal cells.

**Why the Correct Answer is Right:** The half-life of the p53 protein in normal cells is approximately **45 minutes**. This value is derived from studies that measure the rate at which the protein is degraded by the ubiquitin-proteasome system, which is the primary pathway for its degradation.

**Why Each Wrong Option is Incorrect:**

A. **4 hours**: This is an incorrect answer because the half-life of the p53 protein is much shorter than 4 hours (~45 minutes).

B. **15 minutes**: Similar to option A, a half-life of 15 minutes is too short for normal cells.

C. **2 days**: This option is also incorrect as the half-life of p53 protein is significantly shorter than 2 days. The correct half-life is around 45 minutes.

D. **15 minutes**: This is a correct explanation as it matches the correct half-life of the p53 protein in normal cells.

**Clinical Pearl:** The rapid turnover of p53 protein is essential to maintain its functional levels and ensure timely activation in response to cellular stressors, such as DNA damage or oncogenes. A shorter half-life allows for rapid restoration of normal cellular function and prevention of genomic instability.

**Correct Answer:** D. 15 minutes