At sea level, water boils at 100 degrees Celsius under standard atmospheric pressure. More precisely, the familiar value is about 99.97 °C at one atmosphere, so 100 °C is the accepted quiz answer.
Boiling occurs when a liquid's vapor pressure matches the pressure surrounding it, allowing vapor bubbles to form throughout the liquid. Because atmospheric pressure changes with altitude and weather, the boiling temperature is not identical everywhere. At higher elevations, water boils at a lower temperature; at lower pressure, it can boil well below 100 °C.
This is why cooking times can increase in mountain regions: boiling water is cooler there, even though it is visibly boiling. At roughly 1,000 metres above sea level, water boils at about 96.7 °C, while on Mount Everest it boils at approximately 71 °C.
Another source of confusion is the difference between standard atmospheric pressure and the modern SI standard pressure of 100 kilopascals. At 100 kPa, water's boiling point is about 99.61 °C. The Celsius scale was historically defined using water's freezing and boiling points, but its modern definition is tied to the kelvin.