Final answer:
The Islamic caliphate more than doubled in size after Muhammad's death in 632 CE, becoming the largest empire of its time by conquering a large expanse from Spain and Portugal to the Indus River valley. therefore, option D is correct
Step-by-step explanation:
The statement that best describes the territory of the Islamic caliphate in the years following Muhammad’s death in 632 CE is that it more than doubled in size. The caliphate rapidly expanded, taking territory from the Byzantine and Sasanian Empires, growing to include regions as far west as Spain and Portugal and as far east as the Indus River valley in the east, and becoming the world's largest empire of its time. The >Rashidun Caliphate, followed by the Umay_yad and Abbasid Caliphates, each contributed to the expansion and administration of the caliphate, spreading Islam and influencing a vast geographical area. However, managing such a broad empire proved difficult, with various regions declaring independence over time, leading to fragmentation and the rise of successive Islamic states.