What type of wood to replace stair steps?
In that case: Rotted stringer and stair treads...what condition is the landing framework?
With that much rot, I'd consider ripping it all down. Inspectiing the footings (post supports and stringer landing) for proper construction and depth. Usually, older footings are not up to current code.
So, generally, it is best to rip it all down, and start from new, using Pressure Treated Lumber for the framing and stringers, galvanized fasteners, copper flashing, etc...
And one of these for the treads: PT boards, composite material, or a rot & pest resistant wood species.
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