Final answer:
A a. blue star is the hottest among the listed colors of stars, emitting light at shorter wavelengths indicative of higher temperatures compared to red, yellow, or white stars.
Step-by-step explanation:
The color of a star provides a clue about its temperature due to the wavelength of light it emits. The temperature of a star dictates the color of the light it predominantly radiates, and thus the visible color we observe. Blue stars are the hottest, followed by white, yellow, orange, and red stars in decreasing order of temperature. According to Wien's displacement law, hotter objects emit photons with higher energies, which correspond to shorter wavelengths of light. Blue light has a shorter wavelength and higher frequency than red light, indicating that it comes from a hotter source.
Therefore, among the given options, a blue star burns the hottest. This aligns with the fact that a star must be hotter than about 25,000 K to produce an H II region, as both the hottest white dwarfs and main-sequence O stars can do. Thus, the correct answer indicating the hottest star burns with a blue color.