The rapid industrialization of germany most threatened the dominance of which country

The rapid industrialization of Germany most threatened the dominance of the British Empire.
The British Empire was still the biggest super power in the world and the most influential country of all. But Germany started to rise as a competition, both economically and military. That was not taken very fondly by the Brits because they did not wanted to lose their dominance, especially not in their 'front yard', so the relations between the two countries were on shaky ground.

The answer is price.