选D.
A项应改为：Being free,I'd rather read books than sit idle.
B项应改为：I'd rather read books than sit idle in my spare time.
以上两项都考察would rather do sth than do sth.
C项应改为：I prefer reading books to sitting idle in my spare time.考察 prefer doing sth to doing sth.
D项考察prefer to doing sth rather than do sth,是正确的.