Final answer:
Luminosity is the total energy output of a star per second, measured in watts, not to be confused with domestic lighting's lumens per watt.
Step-by-step explanation:
Luminosity is the intrinsic brightness of a celestial object and is measured in watts. This measurement represents the total amount of energy emitted by a star or other astronomical object per second. An analogous concept in domestic lighting is the luminous efficacy, which is quantified in lumens per watt; however, for stellar objects, we focus on the total output without regard to efficiency, hence the use of watts to measure luminosity.
In radio astronomy, brightness is expressed as the amount of energy a radio telescope receives per square meter per second, often in terms of watts per square meter. Therefore, the correct answer is luminosity is measured in A) watts.