What does Sergei mistakenly think?

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C. Beside her bed.

Explanation:

An adjectival phrase, as its names refer, is a group of words that describe a noun or pronoun and act as an adjective.

There is a little trick to identify the adjectival phrases and it is that, usually, the first word with which the phrases begin is whether an adverb or a preposition.

In the question given the option C starts with a preposition (Beside) and the phrase is describing the place where the dress is hanging.

I hope this answer helps you.