Okay, let's tackle this question about the threshold level of herd immunity for Pertussis. First, I need to recall what herd immunity is. Herd immunity is the percentage of a population that needs to be immune to a disease to prevent its spread. The threshold depends on the basic reproduction number (R0) of the disease. The formula for herd immunity threshold is 1 - 1/R0.

Now, Pertussis, also known as whooping cough, is a highly contagious respiratory disease caused by Bordetella pertussis. The R0 for Pertussis is around 12-17. Using the formula, if R0 is 12, the threshold would be 1 - 1/12 ≈ 0.916 or 91.6%. If R0 is 17, then 1 - 1/17 ≈ 0.941 or 94.1%. So the threshold is between 91-94%. I remember that vaccines for Pertussis are the DTaP and Tdap, and high vaccination rates are necessary because of the high R0.

Looking at the options, if the correct answer is around 92-95%, then that's the threshold. The options might include lower numbers like 80% or 85%, which are too low for Pertussis. Other diseases like measles have higher thresholds, around 95%, but Pertussis is a bit lower than that. Wait, no, actually, Pertussis's R0 is similar to measles. Wait, no, measles R0 is higher, like 12-18. So Pertussis's R0 is 12-17. Wait, maybe the threshold is higher? Let me check again. The formula is 1 - 1/R0. For R0=12, 1-1/12= 0.916, which is 91.6%. So the threshold is around 92-94%. Therefore, the correct answer should be in that range.

Now, the incorrect options would be lower, like 80%, 85%, or maybe higher than 95%. For example, if an option says 85%, that's too low. Another might be 95%, which is correct for measles. Another incorrect option could be 70%, which is too low. So the answer is 92-94%, depending on the exact R0 used. The key point is that Pertussis requires a high vaccination rate due to its high R0. The clinical pearl here is that Pertussis has a high R0 and thus a high herd immunity threshold, necessitating high vaccination coverage to prevent outbreaks.

**Core Concept**
Herd immunity threshold is the proportion of a population that must be immune to a disease to prevent sustained transmission. It is calculated using the basic reproduction number (R0) as **1 - 1/R₀**. Pertussis (whooping cough), caused by *Bordetella pertussis*, has an R₀ of 12–17, requiring a high threshold due to its high transmissibility.

**Why the Correct Answer is Right**
For Pertussis, using R₀ = 15, the herd immunity

✓ Correct Answer: C. 90%