Step 1: Recall what an adjective does.
An adjective describes or modifies a noun, giving more information about its quality, quantity, or kind.
Step 2: Analyze each option.
Only functions here as an adverb. Reveal is a verb. Conspiracy is a noun. Meticulous directly describes the noun investigation, telling us what kind of investigation it is.
Step 3: Confirm.
Ask yourself: what kind of investigation? A meticulous one. This question-answer test confirms that meticulous is an adjective in this sentence.
\[ \boxed{\text{meticulous}} \]