Which adjective describes someone who is habitually complaining?

The adjective that describes someone who is habitually complaining is "querulous." This term specifically relates to a tendency to express grievances or criticisms in a whiny or petulant manner. When someone is referred to as querulous, it suggests they frequently voice their dissatisfaction about a variety of issues, often in a way that can be irritating to those around them.

In contrast, the other options convey different meanings. "Complaisant" refers to someone who is willing to please others, often to an excessive degree, rather than someone who is complaining. "Affable" describes a person who is friendly and easy to talk to, which is quite the opposite of being habitually complaining. Finally, "indolent" characterizes someone who is habitually lazy or inactive, rather than someone who is engaged in frequent complaints. This makes "querulous" the most fitting choice to capture the essence of habitual complaining.