Answer: Failing to reject a false null hypothesis.

In hypothesis testing, a **Type II error** occurs when the null hypothesis is false, but we fail to reject it. This is distinct from a **Type I error**, where a true null hypothesis is incorrectly rejected. The options clarify: - **Option A** describes a Type I error, not a Type II error. - **Option B** represents a correct decision, not an error. - **Option C** correctly identifies the scenario of a Type II error, where the null hypothesis is false, but we do not reject it. Understanding these errors is critical in hypothesis testing, as they relate to the concepts of statistical significance and the power of a test.

Author: LeetQuiz Editorial Team