Final answer:
The spectral classification letter that signifies the hottest stellar surface temperature is O.
Step-by-step explanation:
The spectral classification letter that signifies the hottest stellar surface temperature is O. The spectral classes for stars, in order of decreasing temperature, are O, B, A, F, G, K, and M, with each class further subdivided into 10 subclasses numbered from 0 to 9. For example, an O0 star is hotter than an O9 star, and an O9 star is cooler than a B0 star.