Where was the first tank introduced in world war 1?

Prototypes were built in Great Britain during 1915, and a similar process took place in France at roughly the same time. The answer is somewhat complicated, but what the British designers considered the first tank was a prototype they called "Little Willie", in late 1915. However, it did not go into production. Instead a new design was tested, and 150 were ordered in February 1916. The French, who had developed their own tanks quite separately, ordered 400 of their own design two weeks after the British. It was the British who first used tanks in battle, on September 15th, 1916 - 49 were committed, although because of breakdowns only 32 made it into battle.

This is always a tricky question, but, depending on what you mean by the question, the answer is either Little Willie or the British Mark 1 tank. (At the time it was simply called the tank, but when improved versions were introduced it was retrospectively named the Mark 1).