Final answer:
The method of spectroscopic parallax is limited to measuring stellar distances less than 200 parsecs, or approximately 650 light-years, as beyond this range, the precision of the measurements is less reliable.
Step-by-step explanation:
The method of spectroscopic parallax involves estimating stellar distances based on the spectral classification of a star and the apparent magnitude it has in the night sky. To measure the distances of stars using this method effectively, there are limits due to the precision and calibration of the techniques and instruments used. The Hipparcos mission was able to measure accurate parallaxes of stars out to about 300 light-years, which allows us to extend our reach significantly compared to what was possible with ground-based telescopes. Traditionally, ground-based parallax measurements could accurately measure distances up to about 60 light-years. With that in mind, considering the distances stated in the options given as A) 200pc, B) 50pc, C) 100pc, and D) 150pc, and given that 1 parsec is equivalent to 3.26 light-years, the most reasonable limit for traditional spectroscopic parallax measurements would be about 200 parsecs, which corresponds to option A).