To determine which ad language to target to a user, the AdWords system refers to that users:

To determine which ad language to target to a user, the AdWords system refers to that users: 

A)  Chrome Browser setting   
B)  operating system language   
C)  home countrys language   
D)  Google interface language setting

Answer : Google interface language setting

Reason :

What is Google interface language setting?

If you try to communicate with others who don't speak the same language, you might find it tough to get your message across. Similarly with AdWords, you want your ads to appear for customers who can understand them.

Your ads can appear for customers who use Google products and third-party websites in the languages that your campaign targets. This helps ensure that your ads will appear on sites that are written in the language of the customers you'd like to reach.

Read more at https://support.google.com/adwords/answer/1722078?hl=en