The correct answer is - d. At the last glacial maximum, sea level was about 100 meters (300 feet) lower than it is today.
The sea level has always been changing, it has been increasing when the climate has been warmer, and it has been decreasing when the climate has been lower.
During the last glacial period's maximum, which was around 20,000 years ago, the sea level was little more than 100 meters lower than what it is now, some estimates suggest around 120 meters lower. This low sea level enabled lot of places to be connected, like Asia and North America, the islands of Southeast Asia were a single land, the British Isles were merged with Europe, and that helped a lot in the migration of the animals, but also the humans.
The reason for such low sea levels in the glacial periods is that large portion of the ocean waters get frozen at the higher latitudes, thus extracting the water from the oceans and concentrating it as ice further north and south.