Step 1: The WHO defines a sequence: disease leads to impairment, impairment leads to disability, and disability leads to handicap. Impairment is any loss of structure or function (here, the lost leg). Disability is the resulting restriction in the ability to perform an activity in the normal way (here, the inability to walk).
Step 2: The examiner specifically asks about the inability to walk, which is a restriction of activity. This matches the definition of disability, so option b is correct.
Step 3: Impairment (option c) is the loss of the leg itself, not the functional consequence, so it is not the best answer to what is asked. Handicap (option d) is the social or environmental disadvantage that results from a disability and is a wider concept. Disease (option a) is the underlying pathological process, which is absent here.