

  • highest maximum cost-per-click (CPC) bid.
  • most historical data in the account.
  • best Quality Score.
  • highest Ad Rank.

The correct answer is:

  • highest Ad Rank.

Explanation: When several broad match keywords in your ad group broadly match a search term, the system will prefer to use the keyword with the highest Ad Rank.

Read more here: https://support.google.com/adwords/answer/2756257?hl=en