Best Answer - Chosen by Voting
1. The topic marker '은/는' has the sense of "compared to, especially...",
so "오전에는" implies "compared to other times, especially 오전에"
Also it can be used to emphasize "오전에"
2. B is correct.
When talking about "location itself", usually use "에".
That's the reason why the location particle "에" is often followed by "가다(pergi)/있다(ada)/없다(tak ada)".
On the other hand, when interested in "activities at a certain place", then use "에서"
3. I'd say "좋은 하루 보내세요."
\^o^/
