Step 1: The word "quiescent" describes a state of inactivity or calmness, often found in biological or physical contexts.
Step 2: The term that directly contrasts "quiescent" is "active," which denotes something that is operational or engaged.
Step 3: While "dormant" also signifies inactivity or rest, it does not convey the same contrasting meaning as "active" in relation to "quiescent." Additionally, "weak" and "unconcerned" are unrelated and do not serve as valid opposites.
Hence, the correct answer is "active."