Step 1: Understanding the Question:
This question requires identifying the adjective from the given sentence among the provided options.
Step 2: Key Formulas and approach:
An adjective is a describing word whose main syntactic role is to modify a noun or pronoun, giving more details about its qualities, nature, or state.
We must identify which word among the options acts to modify and describe a noun in the sentence.
Step 3: Detailed Explanation:
$\bullet$ Let us check the parts of speech of all options:
$\bullet$ "only" acts as an adverb in this context, modifying the entire noun phrase or limits the scope.
$\bullet$ "reveal" is a verb representing the action of showing or making something known.
$\bullet$ "conspiracy" is a noun, representing the secret plan or plot.
$\bullet$ "meticulous" is a descriptive word meaning "showing great attention to detail; very careful and precise". It directly modifies the noun "investigation", making it an adjective.
Step 4: Final Answer:
Therefore, "meticulous" is the correct adjective, corresponding to Option (D).