What is the correct structure for differentiating between two tasks performed by subjects?

The chosen answer, which emphasizes the phrase "The performance of those who completed the second task," is the most effective structure for differentiating between two tasks performed by subjects. This option clearly specifies that the focus is on the performance aspect related to the second task, thus providing clarity on the distinction being made between the two tasks.

By explicitly mentioning "the performance," it ensures the reader understands that the comparison is not merely about the subjects themselves but also about how they executed the second task specifically. This level of detail is valuable in research contexts, as it delineates the focus of the evaluation.

In contrast, the other options lack the same precision or clarity. Simply stating "leave as is" offers no differentiation, while "those who completed the second task" can seem vague without context regarding the performance aspect. The phrase "as well as those who completed the second task" also introduces ambiguity by bringing in a comparative element without clear distinction or emphasis on performance. Therefore, the choice that articulates a performance-based comparison is the most suitable for the task differentiation required.