Final answer:
The soonest we can expect a reply from a planet orbiting a star 1000 light years away, assuming immediate response, is 2000 years from now due to the time it takes for light to travel such distances.
Step-by-step explanation:
The earliest possible time at which we could hope for a response after sending a radio signal to a star 1000 light years away would be 2000 years from now. This answer corresponds to option (d) "thousands of years from now." This time frame includes 1000 years for our message to travel to the star and, assuming the intelligent life there sends a reply immediately upon receiving it, another 1000 years for their response to travel back to Earth.
Radio signals travel at the speed of light, which is 300,000 kilometers per second. Since a light year is the distance that light travels in one year, and considering the fact that we're attempting communication over an interstellar distance, it's clear that patience is necessary when it comes to potential replies from extraterrestrial civilizations.
Given the tremendous distances involved, real-time communication is currently impossible, and it would take a millennium for our signal to reach the star and another to return with a reply.