Key
O
The sharp shadows and the harsh glare off of the objects point to the key and only light in this picture.
O
The features on one side of the woman's face are completely undiscernable while the other side is bright and clear. The side key light creates this effect.
O
O
O
The key light in this picture is very similar to the previous, although this picture has a source of fill light, shown by the discernable features on the left side of the picture.
The primary source of light in this picture is the sun streaming down through the openings at the top of the building. The light is spilling around the room and reflecting off the floor, illuminating the rest of the cathedral.
The flash from the camera is the key light in this underwater picture. The light from the sun hitting the water at the top doesn't really light up the subject of the picture, so I don't think it's fill light.