Why did j. r.r. tolkien write the hobbit and other stories of middle-earth?

a. he wanted to design a complex imaginary world in which he could set a large number of tales.
b. he wanted to convince readers that middle-earth was real and that the events of his stories truly happened.
c. he wanted to record a factual history of the british isles.
d. he wanted to add to legends that already existed about middle-earth.