Which of the following means "to make" when added to a base word?

The choice that means "to make" when added to a base word is "en." This suffix functions to turn a noun or adjective into a verb that conveys the action of causing or becoming the quality described by the base word. For example, adding "en" to "light" forms the verb "lighten," which means to make light or to become lighter. Similarly, adding "en" to "dark" creates the verb "darken," meaning to make dark. This action-oriented transformation is what distinguishes "en" from the other suffixes, which have different meanings or functions.

The other suffixes, such as "er," typically indicate someone who performs an action or a thing that performs a function, while "ful" conveys the meaning of being full of something, and "less" indicates a lack of something. These distinctions clarify why "en" is the correct choice focusing on the idea of making or transforming qualities.