Which one of these continents lies compltely in the southern hemisphere?

Only two continents lie entirely in the Southern Hemisphere: Australia and Antarctica.

North America and Europe lie entirely in the Northern hemisphere.

South America and Africa both have portions in both North and South hemispheres.

The contiguous land mass of Asia is entirely in the Northern hemisphere, but the island portions of Southeast Asia, such as Indonesia and Malaysia straddle the equator (thus are in both the Northern and Southern hemispheres) and are included with Asia