Which of the following terms is defined as 'instructive' or 'designed to teach'?

The term 'didactic' is defined as 'instructive' or 'designed to teach.' It comes from the Greek word 'didaktikos,' which means 'skilled in teaching.' This term is often used in contexts where the primary purpose of a work, whether it is literature, art, or any form of communication, is to educate the audience or impart knowledge. Didactic texts typically contain lessons or guidelines to follow, effectively serving a teaching function.

In contrast, the other options represent different concepts: 'ephemeral' refers to something that lasts for a very short time, often emphasizing transience; 'subsumed' relates to being included or absorbed within a larger category or context; and 'untenable' describes a position or argument that cannot be defended or supported logically. These meanings do not align with the instructional nature associated with the term 'didactic.'