Find the incorrect part of the sentence:
The old man fell down from the stairs.

Correct option is C

Option (c) contains an error.
Detailed explanation: With stairs, the natural collocation is “fall down the stairs,” not “from the stairs.” The preposition down indicates movement along a vertical/hierarchical path. From typically marks origin/separation and sounds odd here.
Grammatical rule used: Use down to express movement from a higher to a lower position along something (down the hill, down the ladder, down the stairs). Use from to show separation/origin when falling off a distinct surface (fell from the balcony / fell off the chair).
Corrected sentence: The old man fell down the stairs.
Example: She slipped and fell down the stairs, but luckily was not hurt.
Information booster / exceptions:
· Fell off is used when the surface is discrete and you are no longer on it: fell off the ladder/chair.
· Fell from is used with a clear point of origin or height: fell from a great height / fell from the third-floor balcony.