What is the antonym of the term 'plain' when used to describe something clear?

The term 'plain' in the context of describing something as clear suggests a straightforward, unambiguous state. The antonym of 'plain' would logically be a term that implies a lack of clarity or straightforwardness.

When considering the choices, 'obscure' perfectly captures the opposite meaning. It signifies something that is not clear or easily understood, aligning well with the idea of something that is the opposite of plain in terms of clarity. It denotes a condition where meaning or visibility is concealed or difficult to discern.

In contrast, the other options reflect a different focus. 'Clear' reiterates the initial descriptor rather than opposing it. 'Simple' conveys the idea of being easy to understand but does not directly relate to clarity when compared to plain, as something can be simple yet still unclear. 'Transparent' refers to the quality of being easily seen through, which is a positive trait related to clarity, not an antonym to 'plain.'

Therefore, 'obscure' is the most fitting antonym for 'plain' in this context, as it effectively conveys a lack of clarity.