Which of the following is NOT a valid example of a word chain?

A word chain involves a series of words where each word changes only one letter from the previous word. In analyzing the options, it's clear that the correct choice reflects this pattern more than the others.

The option that consists of "plush, lush, lash, last, past" is not a valid example of a word chain because it involves multiple letter changes rather than a single letter change between consecutive words. For instance, transitioning from "plush" to "lush" drops the letter 'p' entirely, which counts as one change but then shifts from "lush" to "lash" by dropping the 'u' and adding an 'a.' This pattern continues with multiple letters changing in each subsequent word, thus breaking the rule of a word chain.

In contrast, the other options maintain a structure where only one letter changes between each consecutive word, making them valid examples of a word chain.