Concept:The word “precarious” means unsafe, unstable, or likely to fall, so the correct opposite must describe something that is safe and firmly fixed.
Explanation:The original sentence states that the terraces were “precarious,” meaning they were dangerous or not securely held.
After the transformation, the walkway should have the opposite quality, that is, something stable and free from danger.
Let us look at each option:
A. “narrow” refers to width, not safety, so it is not an opposite.
B. “smooth” refers to a flat, even surface, which does not directly oppose “precarious.”
C. “durable” means lasting or strong, but it does not necessarily mean secure from falling.
D. “secure” means firmly fixed, safe, and not likely to collapse, which is the direct opposite of “precarious.”
Therefore, “secure” is the most suitable word to fill the gap.
Answer:D. secure