Final answer:
Without specific mass data for all the options, we can't conclusively identify the least massive exoplanet. We know that HD 209458-b is a gas giant with less mass than Jupiter, but without comparative data for TrES-1, OGLE-TR-113b, and HD 149026-b, a definite answer isn't possible.
Step-by-step explanation:
To identify the least massive exoplanet among the given options, let's consider what we know about HD 209458-b. Using the combined Doppler and transit techniques, astronomers determined that HD 209458-b has about 70% the mass of Jupiter but is significantly larger in radius, which is typical of a gas giant. We don't have the direct data available for the other exoplanets in question (TrES-1, OGLE-TR-113b, HD 149026-b) to compare them in the same fashion.
However, based on the principles of the Doppler method and the information provided, we can infer that HD 209458-b, being a gas giant with a large radius despite having less mass than Jupiter, would likely have a lower density than a smaller, rockier planet that could have the same mass. Thus, without the specific mass data for the other planets listed, we can only conclude that if any of the other exoplanets are rocky and smaller in size than HD 209458-b, they could potentially be more massive due to higher density, but without specific measurements for each, it wouldn't be accurate to definitively declare which is the least massive.