обмена

In a sentence

• В университете есть программа обмена студентами.

  The university has a student exchange program.

  Here, "обмена" is in the genitive singular form to indicate the idea of an exchange related to students within the context of the program.

• Мы обсуждаем правила обмена книгами между участниками.

  We are discussing the rules of book exchange among participants.

  In this sentence, "обмена" is also in genitive singular, used to describe the framework or rules governing the activity of exchanging books.

• Центр обмена валютами находится недалеко от станции.

  The currency exchange center is located near the station.

  Here, "обмена" refers specifically to the act or process of exchanging currencies, as a noun in the genitive singular, in the context of a place where this activity occurs.