The purpose of a developer or authentication token is to track:

The purpose of a developer or authentication token is to track:

• AdWords applications.
• Application Programming Interface (API) access by application.
• AdWords usage.
• Application Programming Interface (API) usage.

Correct Answer is:

• Application Programming Interface (API) usage.

Explanation: 

Read more here: https://support.google.com/adwords/answer/2375503?hl=en