Final answer:
Serotonin is involved in causing smooth-muscle contraction and increased gastric motility, aiding in the movement of food through the gastrointestinal system; hence the correct answer is B. Increased gastric motility.
Step-by-step explanation:
Serotonin is a neurotransmitter that is commonly associated with a sense of well-being and is synthesized mostly in neurons that enervate smooth muscle cells in the gastrointestinal tract. Among its roles, serotonin is known to cause smooth-muscle contraction and increased gastric motility, which supports the passage of food in the gastrointestinal tract. Therefore, the correct answer to the question of what effect serotonin has, from the provided options, is B. Increased gastric motility. This is justified by serotonin's role in facilitating the movement of the digestive tract muscles, thereby aiding in the progression of food through the system. Stress is known to impact serotonin production, leading to functional dyspepsia symptoms, as serotonin regulates the expression in smooth muscle cells, indicating the complex interplay between neurotransmitters like serotonin and gastrointestinal function.