**Core Concept**
Whooping cough, also known as pertussis, is a highly contagious respiratory tract infection caused by the bacterium *Bordetella pertussis*. The disease is characterized by a characteristic "whoop" sound made by the patient while inhaling, which is a result of the intense coughing spasm.

**Why the Correct Answer is Right**
The most common source of infection for whooping cough is an individual with a productive cough, who releases infectious droplets into the air. These droplets can be inhaled by susceptible individuals, leading to infection. *Bordetella pertussis* infects the ciliated epithelial cells of the respiratory tract, where it adheres to the cell surface and releases toxins that disrupt normal respiratory function.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not specify a source of infection, making it a vague and irrelevant choice.
**Option B:** This option is incorrect because the primary source of infection for whooping cough is not the feces of an infected individual, but rather the respiratory secretions.
**Option C:** This option is incorrect because the most common source of infection for whooping cough is not the environment, but rather an individual with a productive cough.

**Clinical Pearl / High-Yield Fact**
The most contagious period for whooping cough is during the catarrhal stage, which occurs in the first 2-3 weeks of illness. This is a critical period for diagnosis and isolation.

**Correct Answer: B. Respiratory secretions of an infected individual.**