Final answer:
Most extrasolar planets detected so far have short orbital periods, often less than a week, due to biases in detection methods, especially in data from the Kepler telescope. The mission's 4-year timespan mainly allowed for the discovery of planets within 400-day orbits, with the last year enabling the detection of Earth-like orbits.
Step-by-step explanation:
Based on the data from the Kepler telescope and other observations, most of the extrasolar planets detected to date have c) Short orbital periods, in many cases a week or less. This observational bias stems from a couple of factors. First, larger planets are easier to detect than smaller ones due to their greater effect on their host stars. Second, short-period planets are easier to detect than long-period planets because they transit their stars more frequently, providing more observational data.
Because the Kepler mission had a duration of 4 years, it was mainly able to detect planets with orbital periods of less than 400 days. It wasn't until its final year that it could confirm the presence of planets with yearly orbits similar to Earth's. Therefore, the Kepler discoveries are somewhat limited to shorter orbital periods and sizes larger than Mars. Nevertheless, these findings have led astronomers to extrapolate the data, implying that Earth-size planets may be the most common type of planet in our Galaxy.