Explanation:

Let’s break this down step by step:

- The series is: AXUV, ?, YZWT, XAXS, WBYR.

Here’s what’s going on with the letters:

- 1st letter moves backward by 1 each term: A ? ? ? Y ? X ? W

- 2nd letter moves forward by 1: X ? ? ? Z ? A ? B

- 3rd letter moves backward by 1: U ? ? ? W ? X ? Y

- 4th letter moves backward by 1: V ? ? ? T ? S ? R

Let’s plug in what fits the pattern:

- 1st: After A, to keep with the backward rule, it's Z.

- 2nd: After X, forward is Y.

- 3rd: After U, backward is V.

- 4th: After V, backward is U.

So the missing cluster is: ZYVU

Now, let’s look at your options:

- Option 1: ZYVU  

- Option 2: CZSV – doesn’t fit the rules above.

- Option 3: BZVW – breaks the backward/forward rules.

- Option 4: XWUV – that sequence is out of order.

So, option 1 is the answer.