The correct answer is b) the seminal vesicles.
The seminal vesicles are a pair of glands located behind the bladder that secrete a fluid rich in sugars and other substances. This fluid makes up a significant part of semen and plays a crucial role in activating sperm. It provides the necessary nutrients and energy that sperm need to become motile. While other glands like the prostate also contribute to semen, it is the seminal vesicles that are primarily responsible for this activation process.