Explanation:

When inspection doors on the walls of boilers are opened, the flame does not leap out because of the negative pressure inside the boiler.

Pressure inside is negative:
- The negative pressure inside the boiler helps to keep the flame from leaping out when the inspection doors are opened.
- Negative pressure refers to a pressure that is lower than the atmospheric pressure.
- In a boiler, negative pressure is created by the combustion process and the design of the boiler system.

Reasons for negative pressure inside the boiler:
1. Combustion process:
- The combustion process inside the boiler creates a negative pressure zone.
- As fuel is burned and air is supplied for combustion, the hot gases produced by the combustion process create a vacuum-like effect.
- This vacuum-like effect helps to draw air into the combustion chamber and maintain the flame within the designated area.

2. Boiler design:
- Boilers are designed to have a sealed combustion chamber where the flame is contained.
- The walls of the boiler, including the inspection doors, are designed to be airtight to prevent any leakage of the combustion gases.
- This airtight design, along with the negative pressure inside the boiler, ensures that the flame remains confined to the combustion chamber.

Other options:
a) These holes are small:
- The size of the inspection doors or holes on the walls of the boiler does not directly affect whether the flame will leap out or not.
- Even if the holes were larger, the negative pressure would still prevent the flame from leaping out.

c) Flame always travels in the direction of flow:
- While it is true that the flame in a boiler generally travels in the direction of flow, this does not explain why the flame does not leap out when the inspection doors are opened.
- The negative pressure inside the boiler is the primary reason for preventing the flame from leaping out.

d) These holes are located beyond the furnace:
- The location of the inspection doors or holes on the walls of the boiler does not directly affect whether the flame will leap out or not.
- The negative pressure inside the boiler is what prevents the flame from leaping out, regardless of the location of the holes.