**Core Concept**
Lobar pneumonia is a type of pneumonia characterized by an acute inflammatory process affecting one or more lobes of the lung. It is caused by the invasion of the lung parenchyma by microorganisms, leading to consolidation and impaired gas exchange.

**Why the Correct Answer is Right**
The most common causative organism of lobar pneumonia is *Streptococcus pneumoniae*. This bacterium is a gram-positive diplococcus that colonizes the upper respiratory tract and can invade the lung parenchyma, particularly in individuals with compromised immune systems or underlying chronic conditions. The polysaccharide capsule of *S. pneumoniae* is a key virulence factor, allowing it to evade host immune responses and cause disease.

**Why Each Wrong Option is Incorrect**
* **Option A:** *Haemophilus influenzae* is a common cause of community-acquired pneumonia, but it more commonly causes bronchopneumonia rather than lobar pneumonia.
* **Option B:** *Klebsiella pneumoniae* is a gram-negative rod that can cause severe pneumonia, particularly in individuals with underlying conditions such as chronic alcoholism or diabetes. However, it is not the most common cause of lobar pneumonia.
* **Option D:** *Mycoplasma pneumoniae* is a gram-negative bacterium that causes atypical pneumonia, which is often characterized by mild symptoms and a more gradual onset.

**Clinical Pearl / High-Yield Fact**
Remember that *S. pneumoniae* is a common cause of lobar pneumonia, particularly in individuals with underlying conditions such as chronic heart disease or diabetes. Vaccination against *S. pneumoniae* is recommended for high-risk individuals to prevent invasive pneumococcal disease.

**Correct Answer:** C. *Streptococcus pneumoniae*