The correct answer is: [D]:
_________________________________________________________
"But during solar eclipses, scientists can study the sun's corona."
_________________________________________________________
Note: Refer to the third paragraph of the article. The sun is usually difficult to study. When the sun's corona (which is hotter than the sun's surface) becomes available to study, scientists have a better opportunity to study the corona and try to determine why the corona is hotter than the sun's surface (among other matters).
_________________________________________________________