Step 1: Recall the definition of a noun.
A noun names a:
person,
place,
thing,
idea.
Step 2: Analyse each option. Bridge — a thing; part of the proper noun \emph{Brooklyn Bridge}. ✔
was — a helping verb (form of \emph{be}).
opened — a verb (past participle).
in — a preposition.
Step 3: Identify the noun.
Only Bridge functions as a noun in the sentence.
Hence, the correct answer is
\[
\boxed{\text{Bridge}}
\]