A Type I error occurs when the null hypothesis, H₀, is true but the test decision rejects it. In plain statistical language, this is a false positive: the analysis concludes that there is evidence of an effect, difference, or relationship when in reality the null condition is true. The probability of making a Type I error is denoted by α, the significance level, commonly set at 0.05. A Type II error is different; it occurs when the null hypothesis is false but the test fails to reject it. That is a false negative. Option C is incorrect because rejecting a true null is not a correct decision. Option D describes not rejecting or accepting the null, not the stated event. The critical phrase is “rejecting H₀ when true,” which directly defines a Type I error. Study Guide references/topics: hypothesis testing, null hypothesis, Type I error, significance level.

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