💡 Detailed Explanation
The patterns '〜向け' (muke - targeted at/designed for) and '〜向き' (muki - suitable for) differentiate intentional focus from natural suitability. They connect directly to nouns. Choose '向け' when a product was consciously engineered for a target audience (e.g., children's show), and '向き' for general compatibility or convenience. In this sentence, it translates perfectly as: "This book is written for children.". The correct option '向けに' dynamically matches the structural requirements and logical flow of the statement. The other options do not satisfy the grammatical construction or fail to convey the appropriate semantic nuance.