The correct answer is b$.$ Consecutive measurements are taken. One observes that the system failed to respond only at one, single measurement.

In this scenario, the system is designed to respond to a certain input, but it fails to do so in only one instance. This failure is observed through consecutive measurements, indicating a consistent issue. The other options are incorrect because they do not accurately depict a single point of failure:

a$.$ The technical experts know that the system has at least one defect: This option describes a situation where experts are aware of multiple defects or issues within the system, which is not the definition of a single point of failure.

c$.$ The technical expert realizes that one single defect creates most of the problems: This option suggests that a single defect is responsible for most of the issues within the system, which is not a characteristic of a single point of failure.

d$.$ Only one, single project team member has the required skill: This option does not relate to the concept of a single point of failure. It describes a situation where a team member possesses a specific skill, which is not a defining feature of a single point of failure.

Final answer: Therefore, option b is the correct example of a single point of failure, as it demonstrates a situation where a system fails to respond consistently due to a specific issue.