What contributes the MOST to the United
Kingdom's historical success in international
trade?

Explanation:

I see no options so I will talk about three that I know.

The United Kingdom is on the British Isles which is an island in the North Sea. This made it more accessible for traders as shipping was and still is the main conduit for trade in the world. By manning several ports, the British were able to give access to both incoming and outgoing ships.

From the 17th century, the Royal Navy began to be expanded to enable the British protect its oversees dominions especially the American colonies. The navy were able to protect British ships by offering maritime security such that more British ships were able to trade.

Perhaps the largest contributing factor was the sheer scale of the British empire. At its zenith, the British empire controlled around 24% of the world as we know it. By having the exclusive rights to trade with its empire, the British had more people to trade with and so were hugely successful in international trade.