Obviously, William the Conqueror did not do this by himself. However, the point that this question is trying to make is that the Norman conquest of England did create a new English people.
The Norman conquest did this physically, linguistically and politically. Physically, it brought about a situation where the Normans interbred with the Saxons who were already in England. This changed the look of English people. Linguistically, the Norman conquest brought many new words into the English language, changing it in extremely important ways. You can add to this the importation of new laws that melded with previous traditions to create a new legal system.
By doing these things, the Norman conquest changed England and its people forever, creating a new English people.