Are penguins plantigrade or digigrade?
Plantigrade.


  The penguins are highly specialized for their flightless aquatic
  existence. The feet are located much farther back than those of other
  birds, with the result that the bird carries itself mostly upright;
  its walk can thus be described as plantigrade (i.e., on the soles).
  The sole comprises the whole foot instead of just the toes, as in
  other birds. The most notable characteristic of the group is the
  transformation of the forelimb into a paddle. This is accompanied by a
  body morphology particularly adapted to movement in a liquid medium.
  The thoracic (rib) cage is well developed, and the sternum bears a
  pronounced keel for the attachment of the pectoral muscles, which move
  the flippers. The flipper has the same skeletal base as the wing of
  flying birds but with its elements shortened and flattened, producing
  a relatively rigid limb covered with very short feathers—an ideal
  organ for rapid propulsion. The body plumage likewise consists of very
  short feathers, which minimize friction and turbulence. The density of
  the plumage and the layer of air that it retains provide almost
  complete insulation of the body.


https://www.britannica.com/animal/penguin#ref3467