Select the most appropriate idiom that can substitute the underlined segment in the given sentence.
Popular cosmetics brands in India are very expensive.

The correct answer is cost an arm and a leg.

Let's break down the meaning of the given options to find the most appropriate idiom for the underlined segment "very expensive":

1. Cost an arm and a leg: This is a common idiom used to describe something that is extremely expensive. Substituting it into the sentence gives: "Popular cosmetics brands in India cost an arm and a leg," which perfectly conveys that they are very expensive.
2. Up in arms: This idiom means to be very angry and protesting strongly about something. For example: "The residents were up in arms about the new tax rules."
3. Flog a dead horse: This means to waste energy on a lost cause or a situation that cannot be changed. For example: "Trying to convince him to change his mind is like flogging a dead horse."
4. Take it with a pinch of salt: This means to be skeptical about something and not believe it completely. For example: "You should take his claims with a pinch of salt."

Therefore, the idiom that correctly replaces "very expensive" is cost an arm and a leg.