Final answer:
As of 2016, a few thousand exoplanet candidates have been discovered using the transit method, contributing to a large catalog of potential planets beyond our solar system. This number likely increases as astronomical research and technology progress.
Step-by-step explanation:
As of 2016, the correct answer to the number of planet candidates that have been discovered using the transit method is D) a few thousand. The Kepler mission notably contributed to this tally by identifying a vast number of exoplanet candidates through the transit method. Although it's worth noting that this number continues to grow as new discoveries are made and techniques improve. Not all detections are confirmed exoplanets, but candidates are important data points in the ongoing study of planetary systems beyond our own solar system.