Option 1 -> "be out for" typically means actively seeking something (e.g., "out for revenge"), which doesn't fit the context of experiencing a shock.
Option 2 -> "be on for" is not a standard idiomatic expression that works in this context.
Option 3 -> "be in for" is the correct idiom meaning "about to experience something," often something unexpected or unpleasant.
Option 4 -> "be down for" means "willing to do/try something" in informal speech, which doesn't suit this warning context.
Hence, Option 3: in -> The phrase "be in for a shock" is a common English idiom used to warn someone that they are about to experience something surprising or unexpected. The preposition "in" combines with "for" to create this fixed expression. -> correct