The correct answer is D - X-class
Solar flares are intense bursts of electromagnetic radiation from the Sun.
Scientists separate solar flares into different classes according to their peak brightness in X-ray wavelengths. The least powerful are A-class flares, followed by B, C and M. The highest class of solar flare is X.
Each class marks a 10x increase in energy from the previous one. For example, an X is 10 times stronger than an M flare.
The class is often followed by a number; flares in classes A to M range from 1 to 9, however X flares have no upper limit.
