The correct answer is: B. Under reinforced beam
A balanced beam is a beam in which the depth of the actual neutral axis is equal to the depth of the critical neutral axis. In an under reinforced beam, the depth of the actual neutral axis is more than the depth of the critical neutral axis. This means that the beam is not carrying its maximum load capacity. The beam will be safe, but it will not be as efficient as it could be.
An over reinforced beam is a beam in which the depth of the actual neutral axis is less than the depth of the critical neutral axis. This means that the beam is carrying more load than it is designed for. The beam will be safe, but it will be more likely to crack.
Here is a diagram that shows the difference between a balanced beam, an under reinforced beam, and an over reinforced beam:
[Diagram of a balanced beam, an under reinforced beam, and an over reinforced beam]
The balanced beam is in the middle. The under reinforced beam is on the left, and the over reinforced beam is on the right.
I hope this helps! Let me know if you have any other questions.