Irregular and changing time signatures are a feature of which period?

Irregular and changing time signatures are distinctive characteristics of music in the 20th and 21st centuries. During this period, composers began to experiment with rhythm and meter in innovative ways, moving away from the strict, regular time signatures that were prevalent in earlier music styles. This experimentation led to the use of diverse and complex rhythms, as well as the shifting of time signatures within a single piece for expressive effect.

In contrast, the Classical and Romantic periods typically adhered to more consistent and traditional time signatures. The Baroque period also primarily relied on established rhythmic patterns, focusing more on counterpoint and harmonic development than on altering time signatures. Therefore, the exploration of irregular and changing time signatures is most prominently associated with contemporary music, making the 20th and 21st centuries the correct answer.