3) D

The manager observed in the time period 10:00 to 12:00. We only need to focus on times, which are within this 2-hour period.

Phil:
Entered 9:27 and left 11:03. (Time manager observed = 11:03 – 10:00 = 1 hour 3 minutes)
Entered 11:42. (Time manager observed = 12:00 – 11:42 = 18 minutes)
Total time observed = 1 hour 21 minutes

Quentin:
Entered 11:23 and left 11:46. (Time manager observed =23 minutes)
Entered 11:42 and left 11:55. (Time manager observed = 13 minutes)
Total time observed = 36 minutes

Rob:
Entered 8:20 and left 10:17. (Time manager observed = 10:17 – 10:00 = 17 minutes)
Entered 10:26 and left 11:00. (Time manager observed = 34 minutes)
Entered 11:38. (Time manager observed = 12:00 – 11:38 = 22 minutes)
Total time observed = 1 hour 13 minutes

Sanna:
Entered 9:35 and left 10:10. (Time manager observed = 10:10 – 10:00 = 10 minutes)
Entered 10:16 and left 11:50. (Time manager observed = 11:50 – 10:16 = 1 hour 34 minutes)
Total time observed = 1 hour 44 minutes

Theresa:
Entered 8:44 and left 10:02. (Time manager observed = 10:02 – 10:00 = 2 minutes)
Entered 10:42 and left 12:25. (Time manager observed = 12:00 – 10:42 = 1 hour 18 minutes)
Total time observed = 1 hour 20 minutes

Sanna was in the office for the largest amount of time during the two-hour period. So D is the answer.