The correct answer is **ただちに (tadachi ni)**. In the context of a sentence like 「事故の知らせを受け、彼はただちに現場に向かった」(Upon hearing news of the accident, he headed to the scene immediately), **ただちに** is the most suitable choice. It means 'immediately,' 'at once,' or 'without delay' and is often used in formal or urgent situations. It strongly implies that an action is taken the very moment a stimulus is received, with absolutely no time lag. This fits perfectly with the urgency of responding to an accident report. Common Mistakes / Nuances: ❌ : **やがて** means 'before long,' 'soon,' or 'eventually.' It indicates that some amount of time will pass before something happens. It describes a future event that will occur after a natural progression of time, not an immediate reaction. Therefore, it is incorrect for a situation requiring an instant response. *Example:* 「冬が過ぎれば、やがて春が来る。」 (When winter passes, spring will eventually come.) ❌ : **さっそく** also means 'immediately' or 'right away,' but it carries a specific nuance of doing something promptly because an opportunity has just arisen, often with a sense of eagerness. It's typically used for positive or neutral actions you've been waiting to do. While it denotes speed, **ただちに** is more appropriate for formal, serious, or urgent responses that are duties or reactions rather than seizing a welcome opportunity. *Example:* 「おすすめのレストランを教えてもらったので、さっそく次の日に行ってみた。」 (I was told about a recommended restaurant, so I went to try it right away the next day.) ❌ : **じきに** means 'soon,' 'in a moment,' or 'before long,' similar to やがて but often implying a shorter, more definite waiting period. It still suggests a small gap in time before the action occurs. It's used for things that are expected to happen very soon, but not instantaneously. It does not fit the context of an immediate, split-second reaction. *Example:* 「雨はじきにやむでしょう。」 (The rain will probably stop soon.)

正解は**ただちに**です。例えば、「事故の知らせを受け、彼はただちに現場に向かった」のような文脈では、**ただちに**が最も適切です。これは「すぐに」「間を置かずに」という意味で、フォーマルな場面や緊急性の高い状況でよく使われます。何かをきっかけとして、時間的な隔たりが全くなく、即座に行動を起こすことを強く示唆します。事故の報告に対する緊急の対応という状況に完璧に合います。 Common Mistakes / Nuances: ❌ : **やがて** means 'before long,' 'soon,' or 'eventually.' It indicates that some amount of time will pass before something happens. It describes a future event that will occur after a natural progression of time, not an immediate reaction. Therefore, it is incorrect for a situation requiring an instant response. *Example:* 「冬が過ぎれば、やがて春が来る。」 (When winter passes, spring will eventually come.) ❌ : **さっそく** also means 'immediately' or 'right away,' but it carries a specific nuance of doing something promptly because an opportunity has just arisen, often with a sense of eagerness. It's typically used for positive or neutral actions you've been waiting to do. While it denotes speed, **ただちに** is more appropriate for formal, serious, or urgent responses that are duties or reactions rather than seizing a welcome opportunity. *Example:* 「おすすめのレストランを教えてもらったので、さっそく次の日に行ってみた。」 (I was told about a recommended restaurant, so I went to try it right away the next day.) ❌ : **じきに** means 'soon,' 'in a moment,' or 'before long,' similar to やがて but often implying a shorter, more definite waiting period. It still suggests a small gap in time before the action occurs. It's used for things that are expected to happen very soon, but not instantaneously. It does not fit the context of an immediate, split-second reaction. *Example:* 「雨はじきにやむでしょう。」 (The rain will probably stop soon.)