Let us look at:
Statement 1: At what time Danielle started wouldn't totally help in answering at what time would she finish. So, straight-way, this option ain't sufficient. Eliminate A and D. Although this information might be useful further, let's see...
Statement 2: At 9:30 a.m. Danielle was halfway through the race, and at 10:00 a.m., she was 2/3 of the way through the race.
Let us assume the total distance of the race to be D.
Statement implies that in 30 mins (9:30 a.m.-10:00 a.m.) Danielle covered the distance = (2/3)D-D/2=D/6.
I.E.
1) D/6 distance in 30 mins
2) Danielle had covered D/2 by 9:30 AM, and hence had to cover the next D/2. Using 1) above, D/2 would be covered in 90 mins. Therefore 9:30 AM plus another 90 mins = 11 AM.
So, Danielle will complete the race by 11 AM.
HENCE Statement 2 suffices. Therefore,
answer is B. Hope this was useful.
Happy preparing. Good Luck!