Choose one word that substitutes the highlighted words.

Most adult relationships last for a short period of time. 

Transitory

The appropriate word for the given sentence "Most adult relationships last for a short period of time" is Transitory.

Option 3, Transitory, means temporary or not lasting for a long time, which fits the context of relationships being short-lived.

Explanation of other options:

1. Rotatory: Rotatory relates to rotation or revolving and does not convey the idea of short-lived relationships.
2. Repository: Repository refers to a place where things are stored or kept and is not relevant to the context of relationships.
3. Regular: Regular means occurring at regular intervals or conforming to a standard pattern, which is not the intended meaning in the sentence.