Final answer:
a) Dupin indicates that effective observation requires understanding the unique intellectual methods individuals might use, a concept he values highly and sees as overlooked by the police.
Step-by-step explanation:
In sentence 30, Dupin implies that observation should go beyond surface-level detail and take into account the varied ingenuities of different individuals.
Through the previous passages, it is evident that a) Dupin values detailed observations and considers the intellectual levels and unique methods people might use for concealment, which is often overlooked by others, such as the Prefect.
His insights into human behavior suggest he believes in observing not only the obvious but also the psychologically subtle.