What best describes a testing method that has only one correct answer?

A testing method that has only one correct answer is best described as objective. Objective assessments are designed to measure learners' knowledge or skills through clear and definitive responses. These types of assessments, such as multiple-choice questions or true/false questions, typically have a predetermined correct answer, making it easier to evaluate student performance consistently and fairly.

In contrast, subjective assessments rely on personal interpretations and can include essays or open-ended questions where responses may vary significantly from one student to another. These assessments are influenced by the scorer's views and can lead to more variability in grading.

Formative assessments are aimed at monitoring student learning to provide ongoing feedback that can be used by instructors to improve their teaching and by students to improve their learning during the instructional process. These assessments may or may not have a single correct answer, as they aim to inform ongoing learning rather than to evaluate final performance.

Summative assessments are used to evaluate student learning at the conclusion of an instructional unit by comparing it against some standard or benchmark. While these can include objective questions, they can also contain subjective elements.

Therefore, the characteristic of having only one correct answer aligns perfectly with the definition of objective testing methods, making this the correct choice.