An inselberg is a residual hill with steep sides and usually exposed by denudation. It is a geological landform that stands out conspicuously above the surrounding plain due to its resistance to erosion. Inselbergs are typically composed of hard rocks such as granite, and they are often found in arid or semi-arid regions where weathering is minimal. Therefore, the correct option is: "a residual hill with steep sides and usually exposed by denudation."