Final answer:
The teak tree was considered a "royal tree" by Thai kings and was under strict protection to preserve its valuable wood for significant uses.b) Teak
Step-by-step explanation:
The "royal tree" that Thai kings considered under strict protection is the teak tree (b). Teak has been highly valued for its durability and resistance to extreme weather conditions, making it ideal for shipbuilding, furniture, and other construction requiring sturdy, long-lasting materials. Due to its significance, the teak tree was placed under royal protection to ensure its availability for important uses and to prevent overexploitation.