Which method of determining distance would be most appropriate for a ground-based observer trying to find accurate distances out to approximately 100 parsecs? (choose the lowest applicable rung on the cosmic distance ladder.)

Parallax                

Explanation:

            According to Mark Reid, an astronomer and an astrophysicist at Harvard Smithsonian Center, the Parallax method is the most accurate method to find distance in astronomy as it includes only geometry.

          Astronomers determine the distance of heavenly objects in the space by a method known as the trigonometric parallax or the stellar parallax.

            To measure a very large distances in space, for example the distance between a star or a  planet from Earth, the astronomers uses parallax principle. Here, parallax is defined as a semi-angle made by the inclination between the two sight-lines and the star, and this observation is taken when our Earth is on the opposite sides of  Sun.

Hence the answer is --- Parallax method.

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