In the Tracking Indicators report, which metric identifies how many of the targeted recipients opened...

The metric that identifies how many of the targeted recipients opened a delivery in the Tracking Indicators report is Distinct opens for the population reached. Therefore, option B is the correct answer.

The other options are:

Sum of opens for the population reached: This metric identifies the total number of opens for the population reached.

Distinct clicks for the population reached: This metric identifies the total number of clicks for the population reached.

Recipient clicks for the population reached: This metric identifies the total number of clicks for the population reached, but only counts one click per recipient.