I didn't know the answer but was able to guess using the hints given. Of all the choices, the only hiragana related to the answer was "よう". So what was left was two of the same kanji for day (or sun). And since "よう" needed something more on both sides of it, those kanji were the only choice.
I am wondering if Japanese days of the week are somehow inspired in the same way european day were made. I mean, the SUN- DAY is 日曜日, and 日 means Sun (like in 日本 - Japan, the land of the Sunrise).
And we see the same in MOON-DAY, 月曜日, as 月 means Moon (Tsuki).
In TUES-DAY (Day of Tiw, or Mars), 火曜日, there's not a direct association, but as Mars is the god of war, maybe there is some kind of relation, because 火 is Fire.
日：にち、ひ、び、じつ are the more common readings, with others that are irregular in words such as 明日 (あした).
How it is pronounced depends on the context. Whether or not there is okurigana (hiragana transcription), you should be able to click the audio button to hear how it is used in a particular sentence.