What is the world's smallest ocean?

Answer: 

The Arctic Ocean is the world's smallest ocean. 

According to National Oceanic and Atmospheric Administration,

"With an area of about 5.4 million square miles, the Arctic Ocean is about 1.5 times as big as the United States. It is bordered by Greenland, Canada, Norway, Alaska, and Russia. The average depth of the Arctic Ocean is 12,000 feet and it is 17,850 feet at its deepest point."

Source: https://oceanservice.noaa.gov/facts/smallestocean.html