To monitor the execution time of BigQuery queries, Stackdriver Monitoring is the correct tool as it provides metrics related to BigQuery jobs, including query execution times. Stackdriver Trace is not suitable for this purpose as it is designed for distributed tracing of requests across services, not for measuring the execution time of individual queries. Therefore, the correct action is to use Stackdriver Monitoring to plot query execution times, making option D the correct choice. Options A, B, and C are incorrect for the reasons mentioned.