Which of the following does not yield wool?
A. Yak
B. Camel
C. Goat
D. Woolly dog

Hint: We know that wool is a fiber obtained from animals such as sheep. It is called a natural fiber as it is obtained from animals.

Complete step-by-step answer:
A. Wool can be obtained from yak. Yak wool is smooth, flexible and very dry. Shawls or scarves produced from pure yak wool are difficult to find outside of the Himalayan region. Yak wool is obtained from yak, a longhaired domesticated bovine that lives in the Himalayan highlands. So, option A is not correct.
B. Camel wool is a kind of fiber derived from camel coats. This kind of fabric is more generally referred to as camel fur, and is typically derived from a camel subspecies known as the Bactrian camel. Hence, option B is not correct.
C. Cashmere wool also known simply as cashmere is a fiber derived from different kinds of goats such as, pashmina goats and different breeds of goat of Cashmere. For hundreds of years it has been used for producing yarn, textiles and clothes. So, C is not the correct option.
D. The hair of a wool dog does not provide wool because the dog's hair is unable to spin the same way as that of camel, yak and goat. The spinning of hair is necessary to produce good quality wool.

Hence the correct answer is (D).

Note: Here are different processes in formation of wool, such as, shearing, scouring, grading, dyeing and drying. In shearing, fleece is removed from the animal, in scouring, sheared hair is washed, in grading, the wools are sorted and in dyeing, the sheared wool is dyed and in drying, dyed wool is dried and yarn is obtained.