Concept:The word "precarious" means unstable, risky, or likely to fall, so its opposite must describe something safe, stable, or firmly fixed.
Explanation:The terraces were previously "precarious," meaning they were dangerously unsafe.
The word that fills the gap should show a clear contrast to this danger.
"Secure" means safe, stable, and properly supported, which is the exact opposite of "precarious."
The other options, such as "narrow," "durable," and "smooth," do not directly oppose the idea of being dangerously unstable.
Therefore, "secure" is the most appropriate word to complete the sentence.
Answer:A. secure