Which word means not capable of performing a task?

The word "incapable" directly conveys the idea of not being able to perform a task or lacking the necessary ability or skill to do something. This definition aligns perfectly with the context of the question, which seeks a term that describes a lack of capability.

In contrast, the words among the other options suggest the opposite meaning; "competent," "qualified," and "proficient" all indicate a level of skill or ability to perform a task effectively. Therefore, they do not fit the requirement of describing someone who is unable to carry out a particular task.