Instead of picking a familiar-sounding subset, check each machine against the plain definition of harvesting: removing a mature, ready crop from where it grew.
The cotton picker removes cotton fiber from the plant. The potato digger removes tubers from underground. The mower removes standing forage from the field. The tree shaker removes fruit or nuts from a tree by vibration. Every single one of these four actions is a form of harvesting a crop, just applied to very different crop types and using very different mechanisms.
Since none of the four machines fail this basic harvesting test, all four belong in the answer.
\[\boxed{\text{(A), (B), (C) and (D)}}\]