What creates compound words via central affixes, like in the word speedometer?

The word "speedometer" is formed by combining two distinct elements: "speed" and "meter." In this case, the central affix used to create the compound word is known as an interfix. An interfix serves as a connecting element that joins the two components of the compound without adding any significant meaning of its own.

Interfixes often appear in compound words to facilitate smoother pronunciation and to maintain the structural integrity of the words being combined. In the case of "speedometer," the interfix helps link the two parts, indicating that the meter is specifically measuring speed.

The other options, while related to the study of affixes, serve different functions. Prefixes are added to the beginning of a word, suffixes are attached to the end, and circumfixes involve a combination of prefixes and suffixes surrounding a root word. None of these options apply to the formation of "speedometer" in the same way that the interfix does.