Full Explanation

Sentence Structure & Logic
The sentence sets up a contrast between past and present:

Past: Film development was considered an ______ ritual reserved for skilled technicians-a rite few amateurs dared attempt.
Present: Now hobbyists approach it with casual indifference, even irreverence, thanks to home developing kits.


The key phrase is "a rite few amateurs dared attempt." Why would amateurs not dare attempt it? Because it was difficult, laborious, and demanding. The blank must describe the ritual as hard or challenging.

Blank - arduous

Arduous = requiring great effort; strenuous, laborious, difficult.
Film development was an arduous ritual-so difficult that only skilled technicians could manage it, and amateurs were afraid to try.


This contrasts perfectly with the present: home kits have made it so easy that hobbyists treat it with casual indifference.

Why the Other Options Are Incorrect

A. inviolable - Means sacred, untouchable, or not to be violated. While a "rite few dared attempt" might seem inviolable, the contrast is about difficulty, not sanctity. The present-day shift is from hard to easy-not from sacred to profane.
C. meticulous - Means extremely careful and precise. While film development does require care, the clue is "few amateurs dared attempt"-which is about the barrier to entry (difficulty), not the style of work (carefulness).
D. arcane - Means mysterious, secret, or understood by few. This is tempting because "reserved for skilled technicians" suggests exclusivity. However, the contrast with "casual indifference" is about ease-home kits made it accessible by making it easy, not by revealing secrets. Arduous better explains why amateurs "dared not attempt" it.
E. fleeting - Means brief or short-lived. Completely unrelated to the context.