Final answer:
The royal tree under strict protection by Thai kings is the Teak tree, valued for its durable hardwood and cultural significance.
Step-by-step explanation:
The royal tree that was considered under strict protection by Thai kings is the Teak (A). Teak trees have been highly valued for their durable and water-resistant hardwood. The wood from these trees has been traditionally used in Thailand for constructing everything from royal residences to everyday furniture. As a result of their cultural importance and economic value, teak trees were given special protection by the monarchy, ensuring their preservation and sustainable use.