The correct answer is なだらか (nadaraka). This な-adjective means 'gently sloping,' 'gentle,' or 'smooth.' It is used to describe surfaces that have a gradual, not steep, incline. In the context of a road that is 'とても歩きやすい' (very easy to walk), 'なだらか' perfectly describes a gentle slope that makes walking effortless. Common Mistakes / Nuances: ❌ : This word is incorrect because にこやか means 'smiling' or 'beaming.' It is used exclusively to describe a person's cheerful facial expression and has no relation to the physical shape or slope of a road. For example, 'にこやかな顔' (a smiling face). ❌ : This word is incorrect. しなやか means 'flexible,' 'limber,' or 'supple.' It describes something that can bend easily and gracefully without breaking, such as a dancer's body ('しなやかな体') or a willow branch ('しなやかな枝'). It refers to flexibility, not the gradient of a slope. ❌ : This is a common point of confusion, but it is incorrect here. おだやか means 'calm,' 'gentle,' or 'peaceful.' While it shares the nuance of 'gentleness,' it is used to describe personality ('穏やかな人' - a calm person), weather ('穏やかな天気' - calm weather), or the sea ('穏やかな海' - a calm sea). It is not used to describe a physical incline like a hill or a road. The key difference is that なだらか refers to a physical slope, while おだやか refers to a state of being or atmosphere.

正解は「なだらか」です。「なだらか」は、傾斜が急でなく、緩い様子を表す形容動詞です。「とても歩きやすい」道という文脈において、傾斜が緩やかであることを示す「なだらか」は最も適切な言葉です。 Common Mistakes / Nuances: ❌ : This word is incorrect because にこやか means 'smiling' or 'beaming.' It is used exclusively to describe a person's cheerful facial expression and has no relation to the physical shape or slope of a road. For example, 'にこやかな顔' (a smiling face). ❌ : This word is incorrect. しなやか means 'flexible,' 'limber,' or 'supple.' It describes something that can bend easily and gracefully without breaking, such as a dancer's body ('しなやかな体') or a willow branch ('しなやかな枝'). It refers to flexibility, not the gradient of a slope. ❌ : This is a common point of confusion, but it is incorrect here. おだやか means 'calm,' 'gentle,' or 'peaceful.' While it shares the nuance of 'gentleness,' it is used to describe personality ('穏やかな人' - a calm person), weather ('穏やかな天気' - calm weather), or the sea ('穏やかな海' - a calm sea). It is not used to describe a physical incline like a hill or a road. The key difference is that なだらか refers to a physical slope, while おだやか refers to a state of being or atmosphere.