The organelle that transforms unusable energy in food molecules into a usable form is the mitochondrion.

Here's why:

* Food molecules contain chemical energy, but this energy is not directly usable by the cell.

* Mitochondria are responsible for cellular respiration, a process that breaks down glucose (a sugar) and other food molecules in the presence of oxygen to release energy in the form of ATP (adenosine triphosphate).

* ATP is the primary energy currency of cells, allowing them to perform essential functions like muscle contraction, protein synthesis, and active transport.

So, the mitochondrion is the powerhouse of the cell, transforming unusable energy from food into a usable form that fuels all cellular activities.