Final answer:
The Event Horizon of Cygnus X-1 is 21 kilometers (km). This is the boundary around a black hole beyond which no matter or radiation can return, and it matches the typical scale for such cosmic features, unlike the less likely options of meters, astronomical units, or light years.
Step-by-step explanation:
The Event Horizon of Cygnus X-1 is the point around a black hole where the gravitational pull becomes so strong that nothing, not even light, can escape. To answer the student's question, we must know the size of a light year in meters, which is the distance light travels in one year. From the given reference information, while there is no direct mention of the size of Cygnus X-1's event horizon, we can use relative measures in the solar system and in astrophysics to infer the correct magnitude of measurement. As an astronomical unit (AU) roughly represents the distance from the Earth to the Sun, it is highly unlikely that the event horizon of a black hole would have a size measured in astronomical units. Similarly, it wouldn't be measured in light years as those represent much larger distances typically used to measure the space between stars or galaxies.
Based on our knowledge of black holes and typical values for their event horizons, a more realistic measure is in the scale of kilometers. Therefore, the correct answer for the size of the event horizon of Cygnus X-1 is b. 21 km.