Which word describes something that is boring or unvarying in tone?

The term "monotonous" specifically refers to a lack of variety and interest, particularly in tone or style. It describes something that is tedious or boring due to its uniformity, often characterized by a single tone that does not change in pitch, pace, or intensity. This word aptly captures the essence of something that lacks excitement or variation, making it an ideal choice for describing a dull experience.

In contrast, the other words have distinct meanings that do not align with boredom or uniformity. "Vibrant" suggests liveliness and energy, which is the opposite of dullness. "Diverse" indicates a range of different elements, highlighting variation and richness, while "intriguing" refers to something that captures interest or curiosity, again contrasting the concept of monotony. Thus, "monotonous" is the correct choice as it directly conveys the idea of something being boring or unvarying in tone.