Which suffix indicates a comparative degree or denotes 'one who'?

The suffix that indicates a comparative degree or denotes "one who" is correctly identified as the one that ends in "er." This suffix is typically used in the English language to form adjectives that compare two things, such as "taller" or "smarter," where it signifies that one item has more of a certain quality compared to another.

Moreover, "er" can also be used in nouns to denote "one who" performs an action, as seen in words like "teacher" (one who teaches) or "runner" (one who runs). This dual functionality makes "er” a powerful and versatile suffix in English word formation.

In contrast, the other suffixes serve different purposes: "est" typically indicates the superlative degree rather than a comparative one, showing the highest degree of a quality, such as "tallest." The suffix "ful" conveys the idea of being full of a certain quality, as in "joyful," and "less" is used to indicate the absence of something, as in "fearless." Thus, "er" is distinct in its capacity to create both comparative forms and agent nouns, which is why it is the correct choice in this context.