Step 1: Understanding the Question:
We must identify the part of speech of the bolded word "lest" in the given sentence.
Step 2: Detailed Explanation:
Let us break down the grammatical components of the sentence.
"The thief ran away" forms a complete independent clause with its own subject ("The thief") and verb phrase ("ran away").
"he should be caught" forms a dependent clause with a subject ("he") and a verb phrase ("should be caught").
The word "lest" sits right between these two distinct clauses.
Its grammatical function is to join them together and establish a relationship of negative purpose or fear between the actions.
"Lest" means "for fear that" or "to avoid the risk of".
Words that connect words, phrases, or clauses are called conjunctions. Specifically, "lest" is a subordinating conjunction.
Let us evaluate the incorrect options:
(A) adjective: modifies a noun. "Lest" does not describe any noun.
(B) noun: names a person, place, or thing. "Lest" is not a naming word.
(C) pronoun: replaces a noun. "Lest" does not replace anything.
(E) verb: shows an action or state. "Lest" carries no action.
Because it links two clauses together, it is a conjunction.