The correct answer is: B. Steel provided is insufficient.
An under-reinforced section is a concrete beam that has insufficient steel reinforcement to carry the applied loads. This can lead to the concrete cracking and failing prematurely.
Option A is incorrect because steel can be provided at both the top and bottom of a concrete beam.
Option C is incorrect because steel can be provided on both faces of a concrete beam.
Option D is incorrect because steel will not always yield first in an under-reinforced section. In some cases, the concrete may crack and fail before the steel yields.