What is the transformation to express acknowledgment of past mistakes?

The transformation that effectively expresses acknowledgment of past mistakes is represented in the choice "is true that certain mistakes were." This phrasing directly indicates an acceptance of the reality of past errors, recognizing their existence without minimizing or denying them. It conveys a straightforward acknowledgment that certain mistakes occurred, which is foundational to understanding and learning from past experiences.

The other choices do not carry the same weight of acknowledgment. The option indicating it is "incorrect" implies a denial rather than acceptance of mistakes. The phrase "is clear that" suggests an observation but lacks the component of personal responsibility or admission. Lastly, saying "is regrettable that" introduces a notion of emotion rather than a straightforward acknowledgment, which may dilute the emphasis on accepting past errors. Thus, the choice that most clearly communicates the acknowledgment of past mistakes is "is true that certain mistakes were."