Question

what part of gaia is used to determine the angular positions of stars to better measure their distances and motions?
the radial - velocity instrument
the infrared light instrument
the photometric instrument
the astrometry instrument

Explanation:

Astrometry is the branch of astronomy concerned with precise measurements of the positions and movements of celestial objects. The astrometry instrument on GAIA is designed to measure the angular positions of stars for distance - and motion - related studies. The radial - velocity instrument measures how fast an object is moving towards or away from us, the infrared light instrument is for detecting infrared radiation, and the photometric instrument measures the brightness of stars.

Answer:

the astrometry instrument