Final answer:
The correct answer is 'vociferation', as it denotes loud outcry or shouting, aligning with the description of the baby's cry being so impactful to Omar that it echoes the intensity of his nightmares.
Step-by-step explanation:
Every time the baby cried, Omar heard the vociferation of his nightmares. The term vociferation refers to a loud outcry or shouting, which fits the context of nightmares being distressing and audible. In contrast, 'asperity' suggests harshness or severity, typically of tone or manner, and 'obstinacy' refers to stubbornness or an unyielding attitude. None of these terms convey the loud and overwhelming sense that 'vociferation' does in the context of the baby's cry merging with the terror of nightmares.