Which word would best describe a whimsical idea?

To describe a whimsical idea, the term that best fits is "spontaneous." Whimsicality is inherently playful and imaginative, often characterized by unpredictability and lightheartedness. A whimsical idea typically arises without strict planning or adherence to conventional norms, capturing a sense of creativity and spontaneity.

The other choices do not align with the nature of whimsical ideas. For instance, "rigid" implies strictness and inflexibility, which contrasts sharply with the playful essence of whimsy. "Serious" suggests a level of gravity or solemnity that is not compatible with the lighthearted and carefree spirit of whimsical thoughts. Likewise, "logical" denotes reasoning based on clear principles and structured thinking, which can stifle the imaginative and free-form qualities that characterize whimsy. Thus, "spontaneous" is the most apt choice, as it embraces the unpredictable and playful nature of whimsical ideas.