Which option accurately transforms the statement into a superlative?

The transformation into a superlative form requires expressing the outcome as the highest or best among all possible outcomes in a given context. The phrase "That was the best outcome" directly conveys this idea by using the superlative adjective "best," indicating that, of all outcomes considered, this one stands out as the highest in quality.

In contrast, "That was a good outcome" describes the outcome positively but does not elevate it to the highest level. "That was one of the best outcomes" suggests that there are multiple outcomes equally high in quality, positioning the mentioned outcome among the top but not definitively as the best. "That is my best outcome" presents a personal perspective, claiming it as the best from an individual standpoint rather than as the absolute best in a broader context.

Thus, the correctness of the choice lies in its clear use of the superlative form, which stands alone and powerfully asserts the outcome's superiority.