Let's see what the size of these oceans is:
a. Pacific - around 160 million square kilometers
b. Indian -around 70 million square kilometers
c. Atlantic -around 80 million square kilometers
d. Southern - around 20 million square kilometers
So we can see that the Southern Ocean is a lot smaller than the other oceans: this makes it the best answer; the other oceans are a lot bigger and therefore "major"