The correct answer is `厄介 (やっかい)`. This word means 'troublesome,' 'bothersome,' or 'a nuisance.' It perfectly describes a situation, task, or problem that is complicated, difficult to deal with, and requires a lot of effort. In a sentence like 「この問題は実に厄介だ」 (This problem is truly troublesome), `厄介` accurately captures the difficult and annoying nature of the problem. Common Mistakes / Nuances: ❌ 不便 (ふべん): This word means 'inconvenient.' It refers to a lack of practicality or ease of use. For example, a location far from a train station is `不便`. While a troublesome problem might cause inconvenience, `不便` describes the quality of a tool, place, or system, not the inherent complexity of a problem itself. The nuance is about practicality, not the trouble or difficulty it presents. ❌ 不満 (ふまん): This word means 'dissatisfaction,' 'displeasure,' or 'complaint.' It is a feeling or emotion that a person has. You can feel `不満` *about* a problem, but the problem itself cannot *be* `不満`. The problem's quality is `厄介` (troublesome), which in turn might cause you to feel `不満` (dissatisfied). ❌ 退屈 (たいくつ): This word means 'boring' or 'tedious.' It is used for something that lacks interest, excitement, or variation. A long lecture or a day with nothing to do can be `退屈`. While solving a problem could be a tedious process, `厄介` is a better fit as it specifically focuses on the difficulty and complexity that makes something a hassle, which is the core meaning of 'troublesome.'

正解は「厄介 (やっかい)」です。この言葉は「面倒なこと」「手間がかかって処理が難しいこと」を意味します。例えば「この問題は実に厄介だ」という文では、その問題が複雑で解決するのが大変な、面倒なものであることを的確に表現しています。 Common Mistakes / Nuances: ❌ 不便 (ふべん): This word means 'inconvenient.' It refers to a lack of practicality or ease of use. For example, a location far from a train station is `不便`. While a troublesome problem might cause inconvenience, `不便` describes the quality of a tool, place, or system, not the inherent complexity of a problem itself. The nuance is about practicality, not the trouble or difficulty it presents. ❌ 不満 (ふまん): This word means 'dissatisfaction,' 'displeasure,' or 'complaint.' It is a feeling or emotion that a person has. You can feel `不満` *about* a problem, but the problem itself cannot *be* `不満`. The problem's quality is `厄介` (troublesome), which in turn might cause you to feel `不満` (dissatisfied). ❌ 退屈 (たいくつ): This word means 'boring' or 'tedious.' It is used for something that lacks interest, excitement, or variation. A long lecture or a day with nothing to do can be `退屈`. While solving a problem could be a tedious process, `厄介` is a better fit as it specifically focuses on the difficulty and complexity that makes something a hassle, which is the core meaning of 'troublesome.'