Question

how did we discover how hot the corona was?
both of these
none of these
saw that the ionization of iron with 13 electrons missing
analyzed spectral lines

Explanation:

The high temperature of the solar corona is determined by analyzing spectral lines, which reveal the presence of highly ionized iron (with 13 electrons missing). This extreme ionization only occurs at the millions-of-degrees temperatures of the corona, so both the analysis of spectral lines and the detection of this ionized iron are part of the discovery method.

Answer:

both of these