Step 1: Read both sentences carefully.
“She cannot walk properly.”
“The doctor removes her plaster.”
Step 2: The first action continues up to a point of time when the second action happens.
Step 3: The conjunction “until” is used to show that an action continues up to the time another action occurs.
Step 4: Other options do not fit the meaning:
as shows reason,
when shows time but not continuation,
although shows contrast.
Step 5: Therefore, the correct combined sentence is:
“She cannot walk properly until the doctor removes her plaster.”