**Core Concept**
The question involves the isolation and identification of a bacterial organism from a sputum specimen, focusing on the characteristics of the colonies grown on a 5% blood agar plate. The presence of **a-haemolytic colonies** indicates partial breakdown of red blood cells, which is a key feature in identifying certain types of bacteria.

**Why the Correct Answer is Right**
Given that the colonies are **a-haemolytic**, this suggests the action of an enzyme that partially breaks down the hemoglobin in red blood cells, but not completely, as would be seen with **b-haemolytic** colonies. This pattern is characteristic of *Streptococcus pneumoniae*, a common cause of pneumonia, especially in older adults. The organism's growth on 5% blood agar and its **a-haemolytic** nature are critical clues.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because it does not match the description of the organism's growth characteristics.
**Option B:** Incorrect as it is not typically associated with **a-haemolytic** activity on blood agar.
**Option C:** Incorrect because, although it could be a consideration in respiratory infections, it does not fit the **a-haemolytic** pattern described.
**Option D:** Incorrect as it is not the most likely organism given the **a-haemolytic** colonies on blood agar.

**Clinical Pearl / High-Yield Fact**
Remember, *Streptococcus pneumoniae* is a significant pathogen in community-acquired pneumonia, particularly in the elderly, and its identification through **a-haemolytic** colonies on blood agar is a crucial laboratory finding.

**Correct Answer:** D. Streptococcus pneumoniae