Final answer:
Selective breeding of the Bull Mastiff combined the Mastiff's strength with the Old English Bulldog's friendly nature and protective instincts, giving rise to a dog breed with desirable traits but also potential health concerns due to reduced genetic diversity(option d).
Step-by-step explanation:
The creation of the Bull Mastiff breed through selective breeding is an example of how humans have utilized their understanding of genetic traits to produce desired characteristics in dogs. This process involves selecting specific dogs to produce offspring with particular qualities. For the Bull Mastiff, breeders wanted to combine the protective instincts and physical strength of the Mastiff with the agility and tenacity of the Old English Bulldog. Over several generations, by only breeding dogs that exhibited these traits, the Bull Mastiff breed was developed. This led to a dog breed that is large and strong with a natural wariness of strangers, making them effective as both guard dogs and loyal companions.
Unfortunately, the meticulous focus on breeding for specific traits can lead to a reduction in genetic diversity. This is often detrimental to the health of the breed, leading to an increased risk of conditions like cancer, hip dysplasia, and heart issues. The case of the English Bulldog illustrates such health concerns, where selective breeding for a large head has necessitated cesarian sections for birthing. Therefore, while selective breeding can produce breeds with desirable traits, it can also perpetuate and exacerbate health problems associated with those traits.
therefore, the correct answer is option d: Selective breeding combined the Mastiff's strength and the Old English Bulldog's friendly nature.