Which of the following expressions is an example of an arithmetic sequence?

An arithmetic sequence is defined as a sequence of numbers in which the difference between consecutive terms is constant. This difference is known as the "common difference."

In the first sequence: 2, 4, 6, 8, each term is obtained by adding 2 to the previous term. The common difference here is 2, which confirms that it is an arithmetic sequence.

In the second sequence: 3, 9, 27, 81, the terms do not share a constant difference; instead, they are generated by multiplying by 3. This indicates that this sequence is not arithmetic.

In the third sequence: 5, 10, 15, 20, similarly to the first sequence, each term is obtained by adding 5 to the previous term, hence the common difference is 5. This makes it an arithmetic sequence.

Taking into account the definitions and observations, while the first and third sequences are arithmetic, the second one does not fit the criteria. Therefore, the correct response is not that all options represent arithmetic sequences, as only the first and third do.