Answer:
D. Islam was firmly established in island areas where there was little Hindu influence.
Step-by-step explanation:
Southeast Asia has been a region that has been influenced religiously from the territories west of it for quite some time.
Initially it was the Buddhism and Hinduism that reached Southeast Asia. Both of these religions came from South Asia through the trade routes. The Hinduism didn't really gained lot of followers int he region, unlike the Buddhism. The Buddhism became dominant in the continental part of Southeast Asia, while the Hinduism was only practiced by smaller groups locally.
When the Islam started to spread out, it managed to reach and spread out in Southeast Asia in the same manner, through the trade routes. This religion managed to find its place where the Hinduism and Buddhism didn't really had strongholds, and there were not many followers. The parts of Southeast Asia that accepted the Islam were its southernmost continental part and the Indonesian islands, and the Islam has remained the dominant religion in this places ever since.