Answer:
b). It ensures that measurements are taken from two points which are very far apart.
Step-by-step explanation:
In parallax method of measurement of the distance of far or distant objects we know that we use the method of distance geometry
It is given as

now we have
Arc = distance between two positions from which angle is measured
Angle = total angle subtends by the planet at two positions
radius = distance of the planet
so we will have

so here if we ensure large arc length then the error in the measurement of the angle will be small and the distance will be accurate.