Which of the following is not a major ocean on Earth?

a. Pacific

b. Indian

c. Atlantic

d. Southern

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"