Choosing one of two options; a four-character idiom often used in formal contexts.
経営陣はコスト削減か投資拡大かの二者選一を迫られた。
Management was forced to choose between cost-cutting and increasing investment.
面接での質問は二者選一の形が多く、どちらかを選んで理由を述べるよう求められた。
Many interview questions were presented as a choice between two options, asking candidates to pick one and explain why.
二者選一という言い方は、どちらかしか選べない状況を強調するときに使う。
The expression 'nisha-senitsu' is used to emphasize a situation where only one of two things can be chosen.