Option 1: Flamboyant -> means showy or ostentatious, relates to appearance/style, not an antonym of loud/harsh.
Option 2: Ludicrous -> means absurd or ridiculous, unrelated to the noisy/disorderly meaning of raucous.
Option 3: Shrill -> means high-pitched and piercing, actually similar to raucous rather than opposite.
Option 4: Sombre -> means serious, subdued, and quiet in character, directly opposite to raucous (loud and disorderly).
Hence, Option 4: Sombre -> "Raucous" describes something loud, harsh, and unruly, while "sombre" represents a quiet, serious, and restrained quality, making them clear antonyms -> correct