If you decide to buy a purebred puppy, you would have many different dog breeds to choose from. the wide variety of dog breeds is the result of hundreds of years of
a) random mating.
b) gene sequencing.
c) selective breeding.
d) genetic engineering.

Let's look through the answers: 

Random mating - this is what dogs have done for centuries, and it produces mutts, not pure-breed dogs. 

genetic engineering and gene sequencing are a relatively modern practice, and breeds have existed for a longer time - so this is also not correct. 

the correct answer is selective breeding. This means that the owners mated the dogs with desirable traits and the likelihood that the puppies would have those traits too, increased.