Final answer:
X-ray emissions from the Sun's corona signify its high temperature, which was confirmed by the discovery of spectral lines from highly ionized iron. The corona, with temperatures over a million degrees, emits in high-energy wavelengths, observable by specialized telescopes.
Step-by-step explanation:
The evidence that the corona has a very high temperature is c) Its X-ray emissions. The Sun's corona emits X-rays due to its extremely high temperatures. In the nineteenth century, the observation of an unusual spectral line from the corona suggested the existence of a new element, which was later discovered to be highly ionized iron, indicating temperatures exceeding a million degrees. Besides X-rays, the corona can also be observed in extreme ultraviolet and other high energy wavelengths. The corona's high temperature is why we use instruments like the coronagraph and space telescopes to observe these emissions.
It is important to note that the temperature of astronomical objects significantly influences the type of electromagnetic radiation they emit. This is why objects with temperatures like that of the corona emit in X-ray and ultraviolet spectra.