The correct test is the paired t-test because the experiment uses matched conditions on the same trucks. Each truck receives both tire brands, which means the wear results are naturally paired within the same operating environment. That pairing controls for truck-to-truck variation such as driver behavior, route conditions, load characteristics, suspension differences, and maintenance history. In Six Sigma Analyze Phase work, when two treatments are applied in a matched or dependent way and the goal is to compare mean differences within each matched unit, the paired t-test is the appropriate method. A two-sample t test would be used if the two tire groups were independent, but they are not independent here because both brands are evaluated on the same trucks. Z tests are not appropriate in this situation because population variances are not known and the sample size is relatively small. The paired t-test provides a more sensitive and statistically correct comparison because it isolates the treatment effect from much of the background variation. Therefore, the correct answer is D, a paired t-test.

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