**Core Concept**
The question requires knowledge of microbial bioterrorism and the potential of various biological agents as weapons. Bioterrorism involves the intentional release or dissemination of biological agents to cause harm or death in humans, animals, or plants. The severity of a biological agent as a potential bioterrorism tool depends on its virulence, transmissibility, and ease of production and dissemination.

**Why the Correct Answer is Right**
The correct answer is **B. Bacillus anthracis**. Bacillus anthracis, the causative agent of anthrax, is a gram-positive, rod-shaped bacterium that can be easily produced and disseminated. It has a high mortality rate, especially if not treated promptly, and can be aerosolized, making it a potent bioterrorism agent. The anthrax spores are highly resistant to environmental factors and can survive for long periods, allowing them to be used in a bioterrorism attack.

**Why Each Wrong Option is Incorrect**
**Option A:** Yersinia pestis is the causative agent of plague, a highly infectious and deadly disease. However, its use as a bioterrorism agent is limited due to its relatively short incubation period and the availability of effective treatments.

**Option C:** Clostridium botulinum produces botulinum toxin, a potent neurotoxin that can cause botulism. While it has a high potential for use as a bioterrorism agent, its use is limited by the need for a complex production and dissemination process.

**Option D:** Francisella tularensis is a gram-negative bacterium that causes tularemia. While it has a high mortality rate and can be aerosolized, its use as a bioterrorism agent is limited due to its relatively short incubation period and the availability of effective treatments.

**Clinical Pearl / High-Yield Fact**
The Centers for Disease Control and Prevention (CDC) has designated Bacillus anthracis as a Category A bioterrorism agent, indicating its high potential for use in a bioterrorism attack. This designation highlights the need for vigilance and preparedness in detecting and responding to anthrax outbreaks.

**Correct Answer:** B. Bacillus anthracis