The speaker’s delivery was so utterly _____, a monotone devoid of inflection, that even the most compelling arguments failed to resonate with the audience.

✓ Correct Answer: Option C

A 'monotone devoid of inflection' suggests a dull, unengaging delivery. 'Soporific' means tending to induce drowsiness or sleep, which aptly describes a delivery that makes compelling arguments fail to resonate.