**Core Concept**
The patient's presentation with signs of pneumonia and a gram-positive cocci bacterium showing alpha hemolysis on sheep agar is suggestive of *Streptococcus pneumoniae*, a common cause of community-acquired pneumonia. The alpha hemolysis pattern is characteristic of *S. pneumoniae*, indicating partial breakdown of red blood cells.

**Why the Correct Answer is Right**
*Streptococcus pneumoniae* is identified using the Quellung reaction, a serological test that detects the presence of specific capsular polysaccharides using a polyclonal antiserum. This reaction is highly specific for *S. pneumoniae* and helps confirm the diagnosis. The Quellung reaction is based on the agglutination of bacteria in the presence of a specific antibody, which is a classic example of a precipitin reaction.

**Why Each Wrong Option is Incorrect**
*Option A:* This option is incorrect because the Quellung reaction is not a test for identifying gram-negative bacteria, which do not show alpha hemolysis on sheep agar. *Option B:* This option is incorrect because the Coagglutination test is used to identify *Staphylococcus aureus*, not *S. pneumoniae*. *Option D:* This option is incorrect because the latex agglutination test is used to identify *Cryptococcus neoformans*, not *S. pneumoniae*.

**Clinical Pearl / High-Yield Fact**
The Quellung reaction is a highly specific test for *S. pneumoniae*, but it requires specialized equipment and expertise. In clinical practice, the diagnosis of *S. pneumoniae* pneumonia is often made based on Gram stain and culture results, along with clinical correlation.

**Correct Answer: C. Quellung reaction**