Step 1: Recall the disablement model in community medicine. A pathological event at the organ level produces an impairment, the impairment limits what the individual can do (disability), and that limitation places the person at a disadvantage in society (handicap).
Step 2: Map the clinical facts. Loss of the leg is the structural defect, which is the impairment. Being unable to walk is the lost capacity to carry out a basic task, which is exactly what disability means.
Step 3: The stem highlights the failure to walk, an activity restriction, so the term being tested is disability rather than impairment, handicap, or disease.
\[\boxed{\text{Disability}}\]