Which of the following is an example of an ascender?

An ascender is a part of a lowercase letter that extends above the main body of the letter. In the context of the given options, the letter "k" is the only one that features this characteristic. The ascender is the vertical stroke in the letter "k" that rises above the x-height of the lowercase letters, which serves to provide visual distinction and balance in typography.

The other letters present in the choices do not have ascenders: "g" contains a descender that extends below the baseline, "d" has a portion that extends above the x-height, but it is not as pronounced as that of "k," and "o" is entirely comprised within the x-height. Thus, "k" stands out as the clear example of an ascender among the provided options.