The correct answer is: D. 110 kV
Outdoor substations are installed for voltages above 110 kV for cost and safety reasons. At these voltages, it is more economical to use overhead lines than underground cables. Overhead lines are also less likely to cause damage to property or people in the event of an accident.
Option A: 11 kV is a relatively low voltage, and it is not typically used for outdoor substations.
Option B: 33 kV is a higher voltage than 11 kV, but it is still not typically used for outdoor substations.
Option C: 60 kV is a higher voltage than 33 kV, but it is still not typically used for outdoor substations.
Option D: 110 kV is a high voltage that is typically used for outdoor substations.