Which of the following words means "habitually complaining"?

The word that means "habitually complaining" is querulous. This term describes a tendency to be inclined to complain or find fault with things, often in a childish or petulant manner. It conveys a sense of ongoing dissatisfaction or irritability that can be seen in someone who frequently voices grievances, making it particularly effective in capturing the essence of habitual complaining.

In contrast, garrulous refers to someone who is excessively talkative, often about trivial matters, rather than someone who complains. Fastidious describes a person who is very attentive to detail and precision, sometimes to the point of being difficult to please, but does not specifically imply a habit of complaining. The term perpetual means never-ending or constant but does not relate directly to the act of complaining. Therefore, querulous is the most fitting term for someone who is habitually complaining.