The correct option is C The little boy could not sleep after watching a horror movie.
Adjectives that are placed right before the noun/noun phrase it modifies are attributive adjectives.
Option B is correct as the adjective "little" precedes the noun "boy" that it modifies; the adjective is attributive in relation to the sentence.
Option A is incorrect because the adjective is placed after the noun "movie", attributive adjectives do not immediately follow the nouns that they modify.
Option C is incorrect because the adjective "little" immediately follows the noun "boy", whereas it must precede the noun.
Option D is incorrect because the answer lies in B.