Final answer:
The word that means 'a gradual, subtle process' is insidious.
Step-by-step explanation:
The word that means 'a gradual, subtle process' is b. Insidious.
An insidious process is one that occurs slowly and subtly, often without being noticed or recognized immediately. It can refer to something negative or harmful, such as a disease or a behavior, that gradually and subtly affects a person or situation.
For example, an insidious disease like diabetes may develop slowly and go unnoticed for a long time before presenting clear symptoms. Similarly, an insidious behavior, like manipulation, may take place gradually and subtly, making it difficult to detect.