Final answer:
The small-angle formula relates the angular diameter of an object to its physical diameter and its distance from the observer, which is particularly useful in astronomical observations.
Step-by-step explanation:
The small-angle formula is used in astronomy to relate the angular diameter of an object, as seen from Earth, to its physical size (or linear diameter) and its distance from the observer. So, the correct answer is: A) Its physical diameter and its distance from the observer. The formula is often stated as angular diameter (in arcseconds) = (206,265) * (linear diameter / distance). This formula assumes the angle is small, which is typically the case in astronomical observations where the objects are very far away.
For example, if astronomers want to determine the size of a distant planet, they would use the small-angle formula. By measuring the angle the planet makes in the sky (the angular diameter), and knowing how far away the planet is, they can calculate the planet's actual size.