Identify the sentence in which the underlined clause is a noun clause.
a) the number of competitions has increased every year since sled dog racing was organized.
b) whoever wants to enter into a sled dog race today has a choice of more than 3,000 competitions.
c) although small local races were held in the nineteenth century, a major sled dog race was not staged until 1908.
d) the international federation of sled dog sports, which governs racing, was formed in 1986 to promote the sport.