**Core Concept**
The progression of pneumonia involves several stages, including congestion, red hepatization, grey hepatization, and resolution. Understanding these stages is crucial for diagnosing and managing pneumonia effectively.

**Why the Correct Answer is Right**
Given that the patient has developed sputum expectoration, this indicates the stage where the body is trying to clear the infection, typically associated with the resolution stage, but more specifically, it aligns with the stage of **resolution** or the period after the acute inflammatory phase where the lung starts to clear the exudate. However, sputum production often correlates with the stage after the consolidation phases (red and grey hepatization), which means the correct stage related to sputum expectoration, considering typical progression, would be during or after the resolution phase, but the question seems to aim towards identifying a stage where such symptoms become more apparent, which could be misleading without exact options.

**Why Each Wrong Option is Incorrect**
**Option A:** This would be incorrect as it doesn't directly correlate with the provided information about sputum expectoration.
**Option B:** Similarly, without the exact option text, we can't directly address why it's incorrect, but typically, early stages of pneumonia wouldn't be characterized by sputum expectoration as a defining feature.
**Option C:** Again, lacking the specific option text, but if it refers to an early stage of pneumonia, it wouldn't align with sputum expectoration.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that pneumonia's stages are progressive, and symptoms like sputum production are more characteristic of the body's response to infection and its attempt to clear it, which usually happens after the initial inflammatory and consolidative phases.

**Correct Answer:** D. Resolution stage, considering typical pathological progression and the expectation of sputum production as the body clears the infection.