D.). Radial Method - is the stars, and planets, that orbit around a common center of mass, The star will move towards, and away from earth. Light is shifted, as the star motion changes.
C.). Redshift Method - is described as, how light shifts toward shorter, or longer wavelengths; as objects in space, move closer, or farther away from us. If the star is moving away from us, The light it gets stretched a little, which makes it appear more red than it really is.
B.). Blueshift Method - is the opposite of redshift, its' a decrease in wavelength, due to its' motions towards Earth. If a star is moving closer to us, the light it gives off gets squeezed together, which makes it appear bluer than it actually is.
A.). Triangulation Method - is a method used to determine distances through parallax; The distance equals baseline divides by parallax. It is used to calculate stellar distances by using Earth's orbital diameter, as baseline, the angular displacement of a star is measured.
Therefore, Your answer: Letter Choice (A), Triangulation Method.
Hope that helps, (Answer: Letter Choice (A), Triangulation Method is used to obtain a precise measurement of a star's distance from Earth.).
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