In which sequence are the first term and the common difference equal?

Correct answer: (6, 12, 18, 24, …)

The first term is the initial number of a sequence, while the common difference is found by subtracting each term from the next. In option A, the first term is 6 and the differences are 12 − 6 = 6, 18 − 12 = 6, and 24 − 18 = 6. Hence a₁ = 6 and d = 6, so they are equal. In option B, d = 4, which is not equal to the first term. In option C, d = 0, and in option D, d = −6. Therefore only option A satisfies the required condition. The test must compare the first term with the repeated difference, not merely check whether the sequence is an AP, because all four listed sequences have a constant difference.