Slangs in a language are usually ephemeral in nature.

The correct answer is A. transient.

Slang is a type of language that is used informally and is often specific to a particular group of people. It is often used to create a sense of camaraderie or to exclude outsiders. Slang can be very creative and expressive, but it can also be ephemeral, meaning that it may only be used for a short period of time before it falls out of fashion.

Option B, customary, is incorrect because slang is not always used in a customary way. It can be used in a variety of contexts, and it is not always appropriate to use it in formal settings.

Option C, necessary, is incorrect because slang is not always necessary to communicate. It can be used to add emphasis or to create a sense of informality, but it is not always required.

Option D, obvious, is incorrect because slang is not always obvious to everyone. It can be used by a particular group of people to create a sense of in-group identity, and it may not be understood by outsiders.