Metrics are quantitative measures that can be used to monitor and control various aspects of software testing processes and products1. Metrics can be used to monitor progress along with test preparation and execution by providing information about the status of testing activities, such as test planning, test design, test execution, test evaluation, defect management, etc1. Among the options given in this question, only C is a suitable metric for monitoring progress along with test preparation and execution. The failure rate in testing already completed is a metric that measures how many tests have failed out of the total number of tests executed1. This metric can indicate the quality of the software product under test and the effectiveness of the test cases1. It can also help to identify areas that need more attention or improvement in testing1. Therefore, this metric can be used to monitor progress along with test preparation and execution.