What term describes music that has no strong sense of key and is dissonant?

The term that best describes music with no strong sense of key and that is dissonant is "atonal." Atonality refers to music that does not adhere to traditional tonal centers or key signatures, which means it lacks the hierarchical structure typically found in tonal music where specific notes or chords serve as the focal point. This lack of a central tone results in a sense of freedom from conventional harmonic constraints, allowing for a wider exploration of dissonance and musical expression.

In contrast, the other options relate to different musical concepts. "Modal" refers to music that is based on modes (scale patterns) and can have a tonal center, though not necessarily in a major or minor context. "Monophonic" describes music that consists of a single melodic line without harmonic support, which does not inherently imply a lack of key. "Diatonic" refers to music that is based on the notes within a given scale, typically major or minor, and therefore does have a sense of key and structure. Atonal music stands out because it breaks away from these tonal considerations altogether.