The correct answer is option 2, 15°C.

Option 1, 10°C, is incorrect because the average surface temperature of the Earth is higher than that.

Option 3, 8°C, is incorrect because it is lower than the actual average surface temperature of the Earth.

Option 4, 5°C, is incorrect as well because it is even lower than option 3 and does not reflect the true average surface temperature of the Earth.

The average surface temperature of the Earth is estimated to be around 15°C (59°F). This figure takes into account the temperatures of different regions on Earth, such as the equator and polar regions. The temperature can vary based on factors such as geographical location, time of year, and climate change. It is important to note that this is an average value and specific regions may experience temperatures higher or lower than this average.