Which of the stages of a successful client-trainer relationship includes implementing strategies to improve motivation and promote long-term adherence?

The correct answer is- Action stage.

Explanation:

There are four stages of the client-trainer relationship and that are rapport, investigation, planning, and action. In rapport, a process of maintaining personal relationships and communication with client occurs.

In investigation information on all need of clients are to be gathered. During planning trainer set goals working together with clients. During the action stage, the implementation of all the strategies and programs occurs.  

Progression details, feedback, and instruction are provided according to the need to improve motivation and promote long-term adherence between client and trainer. Therefore the correct answer is action.