In a Reach and Frequency report, “Reach” is defined as the:

• Number of unique users exposed to an ad.
• Demographic populations to which an ad is served.
• Geographic locations in which an ad is served.
• Distance between a user exposed to an ad and the business location of the advertiser.

The correct answer is:

• Number of unique users exposed to an ad.

Explanation: Reach is an estimate of the number of users within a selected location target, based on unique cookies.

You can read more here: https://support.google.com/partners/answer/1722045?hl=en