Step 1: Recall what a noun is.
A noun is a word that names a:
person,
place,
thing,
idea.
Step 2: Analyse each option.
many — a determiner/quantifier, not a noun.
great — an adjective describing the noun \emph{poets}.
placed — a verb (past participle form of \emph{place}).
reserve — a thing/place (library reserve section), hence a noun.
Step 3: Identify the correct noun.
Among the given options, only reserve functions as a noun in the sentence.
Hence, the correct answer is
\[
\boxed{\text{reserve}}
\]