Final answer:
In 1974, the Arecibo radio telescope in Puerto Rico transmitted a signal to M13, a star cluster located about 25,000 light-years away.
Step-by-step explanation:
The correct answer is a) Signal to outer space. In 1974, the Arecibo radio telescope in Puerto Rico transmitted a signal to M13, a star cluster located about 25,000 light-years away. This transmission was a deliberate attempt to make contact with potential extraterrestrial civilizations. The radio signal traveled at the speed of light, which is approximately 300,000 kilometers per second.