Answer:
Dialects are the regional variations of a language.
Step-by-step explanation:
Variety of language that has different vocabulary, grammar or pronunciation is dialect. It differs from one region to the other (of the persons speaking same language but has different dialect). For example, Cantonese in Guangzhou, a city of China to the Mandarin (Standard language of China) is a dialect.
Whereas, slang is an informal form of a language; jargon is the specialized language that is only used and understood by persons in the same profession or group. Idioms are the group of words that delivers meaning that is non-deducible from the individual words. So, Dialects is the right answer.