Your ad can show on the Search Network when someone searches for terms that are similar to your:
D) Ad text
The correct answer is:
Keywords are words or phrases that are used to match your ads with the terms people are searching for. Selecting high quality, relevant keywords for your advertising campaign can help you reach the customers you want, when you want.
When someone searches on Google, your ad could be eligible to appear based on the similarity of your keywords to the person’s search terms, as well as your keyword match types. Keywords are also used to match your ad to sites in the Google Network that are related to your keywords and ads.
Read more here: https://support.google.com/adwords/answer/6323?hl=en