Question E3C07:
Which of the following indicates the greatest solar flare intensity?  

Choices:  
A: Class A  
B: Class Z  
C: Class M  
D: Class X    

Correct Answer is  D: Class X  

To read solar flare intensity, use the GOES X-ray classification. Letters indicate decades of peak X-ray flux in the 1–8 Å band (measured in W/m²), with each step 10× stronger than the previous:

- Class A: $10^{-8}$ to $<10^{-7}$ W/m²
- Class B: $10^{-7}$ to $<10^{-6}$ W/m²
- Class C: $10^{-6}$ to $<10^{-5}$ W/m²
- Class M: $10^{-5}$ to $<10^{-4}$ W/m²
- Class X: $\ge 10^{-4}$ W/m²

Why D (Class X) is correct:
- Class X is the highest category; it indicates the greatest solar flare intensity.

Why the others are incorrect:
- A (Class A): Weakest flares, far below X by factors of $10^3$ to $10^4$ or more.
- B (Class Z): No such class exists; “Z” is not part of the GOES scale.
- C (Class M): M-class is strong but one decade lower than X-class.

Useful rules of thumb:
- Each letter step (A → B → C → M → X) is 10× increase in peak X-ray flux.
- The number after the letter scales within the class. Example:
  - M5 means $5 \times 10^{-5}$ W/m²
  - X2 means $2 \times 10^{-4}$ W/m²
- X-class flares are the ones most likely to cause significant HF radio blackouts.