The correct answer is: A. Innermost region
The innermost region of a flame is the hottest, and is therefore used by goldsmiths to heat gold. The middle region of the flame is less hot, and is used to melt wax or solder. The outermost region of the flame is the coolest, and is used to prevent the gold from overheating.
The innermost region of a flame is called the combustion zone. It is the hottest part of the flame, and is where the fuel and oxygen react to produce heat. The middle region of the flame is called the reducing zone. It is less hot than the combustion zone, and is where the carbon monoxide produced in the combustion zone reacts with the oxygen in the air to produce carbon dioxide. The outermost region of the flame is called the oxidizing zone. It is the coolest part of the flame, and is where the oxygen in the air reacts with the fuel to produce carbon dioxide.
Goldsmiths use the innermost region of the flame to heat gold because it is the hottest part of the flame. The heat from the innermost region of the flame is sufficient to melt gold, which is a very dense metal. The middle region of the flame is less hot than the innermost region, and is therefore used to melt wax or solder. Wax and solder are both less dense than gold, and therefore require less heat to melt. The outermost region of the flame is the coolest part of the flame, and is used to prevent the gold from overheating. If the gold is heated too much, it can become brittle and break.