Final answer:
The dividing line between Gander and Shanwick Oceanic air traffic control regions is at the longitude of 30W.
Step-by-step explanation:
The dividing line between Gander and Shanwick Oceanic is a segment of the North Atlantic which marks the boundary between the air traffic control regions of Gander Oceanic, controlled by Canada, and Shanwick Oceanic, controlled by the United Kingdom. This boundary is defined by the longitude of 30W, and hence the correct answer to the question 'What is the dividing line between Gander and Shanwick Oceanic?' is c. 30W.