On witch thickly clouded planet has radar mapping revealed a varied topography consisting plains highlands and thousands of volcanic structures?

The planet you are referring to is Venus. Radar mapping conducted by missions such as NASA's Magellan has revealed its varied topography, which includes extensive plains, rugged highlands, and numerous volcanic structures. The thick clouds of sulfuric acid that envelop Venus obscure visible light, making radar an essential tool for studying its surface features.