The environment of Arabia served as a form of protection during much of the region's history primarily due to its extreme heat and arid conditions. These harsh environmental factors made it challenging for large armies to traverse the region, thereby deterring invasion and protecting local settlements. The desert's inhospitable nature acted as a barrier, discouraging potential invaders who would face difficulties in sustaining themselves in such an environment.
Additionally, while the mountainous regions of Arabia could also provide some protection, the most significant deterrent for invaders throughout history was the overall harshness and aridity of the landscape, which limited movement and access to resources. Therefore, the correct answer to how Arabia's environment offered protection is primarily linked to its extreme heat and arid conditions, rather than the other options listed.