The answer is phlegmatic.
A melancholic person, according to Galen's theory of the four temperaments, is typically described as thoughtful, introspective, and often prone to sadness or depression. While "thoughtful" might apply, "calm" and "reliable" are not the primary characteristics associated with melancholic individuals.
Choleric individuals are characterized by being ambitious, energetic, and passionate. They are often seen as leaders but can also be easily angered and impatient. These traits do not align with being calm, reliable, and thoughtful.
Phlegmatic individuals are described as calm, reliable, and thoughtful. They are typically relaxed, peaceful, and consistent, making this the best match for the description provided.
Sanguine people are generally sociable, lively, and carefree. They are often described as being optimistic and fun-loving, which does not align with the calm and thoughtful characteristics mentioned.